why not access much than 8K of the sdram?

working with xilinx edk. I have a sdram of 32M, when I tried to copy data from flash to sdram, i works well only when the accouts is less than 8K, when i copy data which is more than 8K,it is wrong. it just repeatly copy a little data t sdram,what is the problem should be? I am sure my flash works well.