USRP MIMO-OFDM cannot receive packet

Recently,I build the 2x2 mimo-ofdm system with only mimo cable.The
transmitting end can successful the packet,while the send end can only
do coarse freq1 and freq2 estimation,sometimes could receiver one or two
packet and not get the successive packages of transmitting end.I don’t
know what problem happened. Could anyone points out the problem for me?
If It is synchronization issue.