Multiple READ and WRITE controllers to SDRAM with AVALON BUS (No Nios)

I assume you are using these: http://www.altera.com/support/examples/nios2/exm-avalon-mm.html?gsa_pos=1&wt.oss_r=1&wt.oss=template

The write buffer in that template will become full if data continues to enter into it while the write master is waiting. It could be waiting because your controller hasn't told it to move any data or the arbiter for the memory hasn't let it continue (something else is accessing the memory).

You might want to use a DMA for what you are doing. The data from the camera could be formed into Avalon-ST using a bridging component and then the DMA could take the streaming data and pipe it into memory. Likewise the same could happen in reverse for sending data back to the outside.