Does anyone have any experience with rakwireless PLC boards or any other low voltage powerline communication systems?

I work for a company the installs internet connected instruments underwater. Recently we started to work with power line communication to increase our potential range. However, we are see some odd behavior. We are able to get good communication between the modules when on the bench but get intermittent failure when using a longer cable. For example: we connected two units together with a 2 conductor, 16 gauge, 1000ft SOW cable rolled up on a spool. The units failed to communicate. So, we started to unroll by ~100 ft lengths, again and again, and retested each time. One of the guys in the shop is convinced the coil is the problem. The units wouldn't communicate until we got the whole cable off the spool. As an experiment we re-rapped about 50ft on the spool and tested again, it didn't work. Slowly we removed the cable from the spool trying to prove it was the spool causing the problem and find the number of coils that cause failure. Well, even after we got the cable off the spool again the units still wouldn't communicate! To make things worse, we re-rapped the whole cable and took the spool of cable and the two units to our engineer to show him the issue and with the cable on the spool the two units communicate just fine... We bring everything back to the shop, hook it up again, and nothing...

We are running 48v through the SOW cable with the comms, and it powers up the unit on the other side so we know things are hooked up correctly (continuity wise). Its just inconsistent as to whether the two will start up the communication between the two units.

Any ideas on what we might be going wrong? Is rakwireless hardware any good, I haven't been able to find any good guides or reviews for their stuff?

Thanks in advance!