Consecutive Grid

I have a problem with the use of Grids, In fact I use two grids to do two different computing(kernels), but second depend on the first kernel results. When I compare result, I found that results of the second kernel are not correct, and they have as difference a multiple of 2!!!
If I use this kernel alone, results are correct!
Any one has an idea?

Note: I dont move data from GPU memory between executions.