Value is not updated when ChainCode is executed - hyperledger-fabric

Question

I am following a tutorial for hyperledger-fabric, below is the link.
http://hyperledger-fabric.readthedocs.io/en/latest/build_network.html.
When I executed byfn.sh as per instructed by the link, it fails when trying to query the value after the invoke to transfer 10 value from a to b is executed. The query result is still 100 and not 90 despite the Success Response : 200 and no error returned from the ChainCode.
I've also proven this by going to CLI container and executed the command step-by-step.
I wonder if any of you faced this issue and moreover if there's any guidance how to debug this (e.g. how to see the chain in the blockchain, what log to check if the value fails to be updated but the response is success, etc.).
Thanks in advance
[Edit]: From the instruction (http://hyperledger-fabric.readthedocs.io/en/latest/build_network.html), it is mentioned that we need to run (also part of byfn.sh)
peer chaincode invoke -o orderer.example.com:7050 --tls --cafile /opt/gopath/src/github.com/hyperledger/fabric/peer/crypto/ordererOrganizations/example.com/orderers/orderer.example.com/msp/tlscacerts/tlsca.example.com-cert.pem -C $CHANNEL_NAME -n mycc -c '{"Args":["invoke","a","b","10"]}'
to transfer the value from 'a' to 'b', and afterwards when we invoke a query to peers, the value of a should change to 90 from originally 100.
peer chaincode query -C $CHANNEL_NAME -n mycc -c '{"Args":["query","a"]}'
The chaincode installed to each peer can be found here: https://github.com/hyperledger/fabric-samples/blob/release/chaincode/chaincode_example02/chaincode_example02.go
Copy-paste the code that invokes to update the value
func (t *SimpleChaincode) invoke(stub shim.ChaincodeStubInterface, args []string) pb.Response {
var A, B string // Entities
var Aval, Bval int // Asset holdings
var X int // Transaction value
var err error
if len(args) != 3 {
return shim.Error("Incorrect number of arguments. Expecting 3")
}
A = args[0]
B = args[1]
// Get the state from the ledger
// TODO: will be nice to have a GetAllState call to ledger
Avalbytes, err := stub.GetState(A)
if err != nil {
return shim.Error("Failed to get state")
}
if Avalbytes == nil {
return shim.Error("Entity not found")
}
Aval, _ = strconv.Atoi(string(Avalbytes))
Bvalbytes, err := stub.GetState(B)
if err != nil {
return shim.Error("Failed to get state")
}
if Bvalbytes == nil {
return shim.Error("Entity not found")
}
Bval, _ = strconv.Atoi(string(Bvalbytes))
// Perform the execution
X, err = strconv.Atoi(args[2])
if err != nil {
return shim.Error("Invalid transaction amount, expecting a integer value")
}
Aval = Aval - X
Bval = Bval + X
fmt.Printf("Aval = %d, Bval = %d\n", Aval, Bval)
// Write the state back to the ledger
err = stub.PutState(A, []byte(strconv.Itoa(Aval)))
if err != nil {
return shim.Error(err.Error())
}
err = stub.PutState(B, []byte(strconv.Itoa(Bval)))
if err != nil {
return shim.Error(err.Error())
}
return shim.Success(nil)
}
I am pretty confused how it can fail to update the value and if someone can give me a hint how to debug the blockchain or fabric, that will be great.