RFBee corrupted, not working

Hello All,
I was working on multiple RFBees, 1 receiver node and 4 sending nodes and, sending data at a very low packet interval of 30ms [Duration between each packet sent from a node]. I sent around 2000 packets from all 4 nodes simultaneously and noticed that after sometime, receiver RFBEE wasn’t working. Even it didn’t let me burn a new firmware in it and it happened to around 8 RFBees, in a same pattern.

Strange thing is that sender RFBees are still working.

Can anyone help me out what problem could be this, and how can I recover my RFBEEs.