reading data from sram

hi:

now,I have written a static image frame into the sram, and then I want to read the frame from the sram all the time,in signaltapII ,according to the reading and writting wave,I find that when reading data from the sram at the first time ,the reading data is equal to the writting data,but when readding at the after times,the data goes wrong,I donnot understand the reason?

I have post a short cut of the reading and writting wave,in the read wrong.bmp,the data is not equal to the writting data,some times are all zero ,and sometimes are other data.

I hope some one could give me some possible reasons,thanks a lot!