When using the "to_fifo" shared memory block, System Generator does not pass the correct 'input_depth' and 'output_depth' parameters to Coregen if the depth chosen from the drop-down box is greater than 512. For instance, instead of passing 1024, 2048, etc. it passes 1000, 2000, etc. this results in an incorrect implementation.
The workaround is to use a deeper fifo. To do this you can disable the link to the library, edit the mask of the "to fifo" block, and then edit the various popup values via the parameters link. Change "1K" to "1024", "2K" to "2048", etc.
This has been fixed in System Generator for DSP 7.1.