Delphi CPU Usage low during MultiThreading - multithreading

Question

During Executing of a Multithreading program I see 8 threads starting in the Delphi Event log.
(My CPU is a Intel 7 with 4 cores HyperThreaded so 8 Calculation Cores) but in my TaskManager at the Performance tab I see only 12% CPU usage and only one core calculating with performance up to about 70 - 80 %. Did compile my multithreading program with OTL usage and with ParallelFor usage, But still only 12% performance and only One core doing the work.
On my Form1 I have a ButtonClick procedure with the OTL parallel.ForeEach which iterates over items of a StingList.
The StringList lines contains each a Name, a Path to a differend DataFile and a DataFormat of the File.
The ForEach.execute() Starts a 'EntrySearch'procedure on other Unit,
The EntrySearch procedure starts with the extraction of the info from the appropiate line of the Stringlist.
In a 'While X < Y loop' is the data extracted from the DataFile through a AssignFile and While not eof,read the lines with data. Calcuclations are made on the Data until the 'While X < Y' loop ends
I can see that 8 (CPUcount) Threads are started at the ButtonClick procedure. In the TaskManager I see only one CPU core start working total of about 12% ProcessorUsage.
When after the calculations the ProcessorUsage returns to 0% the .exe program is hanging and I have no controlover the program.
From the little data I can extract out of the CalculationUnit I getonly data from the last started thread, as of this last thread makes the other threads stop and cannot make their caculations and can not terminate.
{the OTL in the ButtonClick procedure}
Parallel.ForEach(0, StrList.Count-1)
.PreserveOrder
.NumTasks(CPUCount)
.NoWait
.Execute(
procedure(const value: integer)
begin
CalcUnit.EntrySearch(value);
end);
{procedure on CalcUnit}
procedure EntrySearch(value: integer);
begin
{extract Name, Path DataFile and DataFormat from StringList}
While X < Y do begin
AssignFile(qMSInputFile7, Path);
{$I-} reset(qMSInputFile7); {$I+}
While Not eof(qMSInputFile7) do Begin
with qMetaRec7 do begin
Read (qMSInputFile7, qMetaRec7);
{ Extract the Data}
end; // While not eof
{Make calculations}
end; // While X<Y
end;
What goes wrong? and how can I solve this.
Thanks A lot.