Troubleshooting Pi is not receiving data with 2 arduino nodes

Hi There,

I got an odd thing just popped up with my setup:

one Pi/Emonhub as the gateway, with two EmonTx V3. I've modified one of these 2 EmonTxV3s to to use nodeID 9, and left the other one as 10. When I initially started them up, they are all working just fine as I can see the PI was receiving data from both nodes every 10 - 11s, however, after 3 days, all the sudden I noticed that I got significant less value from node 10 compare to 9. after that, I checked the log in Pi and it was indeed the case.

Now my questions is, they both worked fine initially therefore I don't really think two nodes are interfere each other. and cause any package loss. My next thought is, at this moment, only node 9 works normally, but 10 is sending less data leads me to think that there are some issue with the Node 10 emonTx box. I plugged in my computer to monitor the serial output from the node 10 emonTx box and it looks like I am getting serial output everything 10-11s. That's why I am starting to suspect that the RF module got some issue, even though, the serial output shows, the sketch just works as expected. 

Any thoughts on this issue and how can I troubleshoot further?