VBA code should capitalize first letter in sentence, but doesn't - excel

Question

I'm unsure why the following line of code is failing to capitalize the first letter of every first sentence in a cell:
chars.Characters(1, 1).Text = UCase(chars.Characters(1, 1).Text)
It does work on most of the cells, but about 1 in 6 do not.
Basic troubleshooting I attempted: 1) Made sure no spaces before the first letter, 2) Used regular UPPER formula successfully on same cell, 3) Re-typed the word, 4) Tried a different word.
Full code in case it helps:
Sub ColorWords()
Dim sh As Worksheet
Dim cell As Range
Dim chars As Range
Dim defRng As Range
Dim wordRng As Range
Dim posB As Long
Dim posE As Long
Dim posQ As Long
Dim color As Long
Dim word As String
Dim def As String
Set sh = Sheets(1)
Set defRng = sh.Range("D3", sh.Range("D1048576").End(xlUp).Address)
Set wordRng = sh.Range("C3", sh.Range("C1048576").End(xlUp).Address)
Application.ScreenUpdating = False
For Each cell In wordRng
word = LCase(cell.Value)
If IsNull(cell.Font.color) Or cell.Font.color = 0 Then GoTo NextIter
color = cell.Font.color
For Each chars In defRng
chars.Characters(1, 1).Text = UCase(chars.Characters(1, 1).Text)
def = LCase(chars.Value2)
posB = InStr(1, def, word)
If InStr(posB + Len(word), def, "s") = posB + Len(word) Then
posE = Len(word) + 1
Else
posE = Len(word)
End If
Do Until posB = 0
chars.Characters(posB, posE).Font.color = color
chars.Characters(posB, posE).Font.Bold = True
posB = InStr(posB + 1, def, word)
If InStr(posB + Len(word), def, "s") = 1 Then
posE = Len(word) + 1
Else
posE = Len(word)
End If
Loop
Next chars
NextIter:
Next cell
Application.ScreenUpdating = True
End Sub`

Answer 1

I found the logic of your code inconsistent, at times to the point of confusing, and added comments to my substitute to explain. The code below will loop through all phrases in column D, capitalize their first word and remove any trailing or leading blanks.
It will then, one phrase after the other, look for existence of each word in column C. If found, the word will be given the colour it has in column C, including any trailing "s". I hope that's largely what you wanted. However, if there are small differences you may like to take the syntax I provide and move it to the location in the code where it is needed.
Sub ColorWords()
' 124
Dim wordRng As Range ' words to look for
Dim defRng As Range ' subject phrases
Dim wordCell As Range
Dim defCell As Range
Dim posB As Long ' start of word
Dim posE As Long ' length of word
Dim Color As Long
Dim Word As String
Dim Def As String
With Worksheets(1) ' change to suit (suggest use of a name)
' syntax for specifying a cell: Cells([Row number], [Column number or name])
' syntax for specifying a range: Range([First wordCell], [last wordCell])
Set wordRng = .Range(.Range("C3"), .Cells(.Rows.Count, "C").End(xlUp))
Set defRng = .Range(.Range("D3"), .Cells(.Rows.Count, "D").End(xlUp))
End With
Application.ScreenUpdating = False
' All colours disappear upon capitalisation.
' Therefore call each defCell only once.
For Each defCell In defRng
With defCell
Def = LCase(Trim(.Value2)) ' remove leading/trailing blanks
Def = UCase(Left(Def, 1)) & Mid(Def, 2)
' your code first capitalises the first letter, then
' makes the entire phrase lower case, including its first letter
.Value = Def
.Font.Color = 0 ' remove existing colors
End With
For Each wordCell In wordRng
With wordCell
Word = Trim(LCase(.Value)) ' remove leading/trailing blanks
Color = .Font.Color ' can't be Null
' Font.Color is of Long data type, which is 0 if not set
End With
If Color <> 0 Then ' 0 is the default font color (usually black)
' skip if Word's colour is the default
' vbTextCompare = case insensitive, else vbBinaryCompare
posB = InStr(1, Def, Word, vbTextCompare)
posE = Len(Word)
If Mid(Def, posB + posE, 1) = "s" Then posE = posE + 1
With defCell.Characters(Start:=posB, Length:=posE).Font
.Color = Color
.Bold = True
End With
End If
Next wordCell
Next defCell
Application.ScreenUpdating = True
End Sub
Although the code looks at the character following the "word" I refrained from adding action to avoid colouring the word "text" in "textbook". This might easily be added if you prefer to either skip "textbook" or expand the colour to include the entire word.
Note also that I added a line of code to remove all colours from a phrase before making changes to it. This enables running of the same program on the same phrases repeatedly when words or colours in column C are modified.

Answer 2

I'm not sure what you are doing with the colors, but to capitalize the first letter of each sentence with in a cell, you can use Regular Expressions to determine where that letter is, and the Characters property to change the case to Upper.
eg:
Note that I used early-binding so as to make use of intellisense, and it's a bit more efficient, but you can convert to late-binding if you need to
'Set Reference (Tools/References) to Microsoft VBScript Regular Expressions 5.5
Option Explicit
Sub capFirst()
Dim R As Range, C As Range
Dim RE As RegExp, MC As MatchCollection, M As Match
Dim ch As String
Const sPat As String = "^.*$"
With ThisWorkbook.Worksheets("sheet1")
Set R = .Range(.Cells(1, 1), .Cells(.Rows.Count, 1).End(xlUp))
End With
Set RE = New RegExp
With RE
.Pattern = sPat
.MultiLine = True
.Global = True
For Each C In R
If .Test(C.Value2) Then
Set MC = .Execute(C.Value2)
For Each M In MC
C.Characters(M.FirstIndex + 1, 1).Text = UCase(C.Characters(M.FirstIndex + 1, 1).Text)
Next M
End If
Next C
End With
End Sub
I imagine that whatever is going on with your colors could be integrated into the above.