Hi Nimitt,
I think manual entry will take precedence to substitution rule if I am not mistaken. I remember facing this issue long time back. So, check these options of substitution rule as well, but if manual entry is taking precedence as I suspected, then try to remove the field from entry screen using field status groups or account assignment templates. Then user will not enter any different profit center while posting the document. Default PC maintained in FAGL3KEH will be taken by the system.
Sridhar