What is your goal?
It is normal that a implemented design changes between builds because placement relies on a random seed. If the source code, constraints or seed change, then the final result will be different to a previous build.
Are you assuming that because the implementation has changed your design no longer works? If your design uses fully synchronous design and is properly constrained, if the design meets timing then it should work every time. If it is not working, I suggest there is a problem with the code or lack of timing specs. Using pblocks may help only by masking the problem in specs or source code.
So, what exactly is the problem (pblocks are not usually the answer to anything).