If the host doesn't read the data, the FX3 buffers will get filled and no more data will be read. The host need to ask the data, thus freeing the buffers for next read.The flags indicate the availability of the buffers, in this case, it all got full. So, what you see on the flag, is an expected behavior: it is just indicating the master that it is not ready to take anymore data as the buffers are full. Please refer to AN65974, it describes in details the steps to test the slave-fifo functionality. Let us know for any further query!