Deployment planning wrong in the first bucket

Hi,

I am working on SCM 7.0 where simple heuristics and deployment is running in month bucket. When deployment is run after heuristic, the first distribution demand (P) created by SNP bucket moves backward by a month or 2 but after this  period it is planning correct. I checked couple of things master data,  Product location, Deployment profile, lead time, location calender etc but did not find the issue. Can any one help me to resolve the issue please.