Mike from FTL

I desperately need some help. I have an ISY-994i and a PLM connected for a few years and I'm pretty tech savy and have been pleased up until now. It works great EXCEPT I can no longer add any new modules. Every time I try to add a new one it tries to about 7% and then says "Cannot Determine Insteon Engine". I just bought a new PLM and replaced it because I thought it might be the problem but the same problem still occurs. I've been through a couple troubleshooting articles but none seem to have an answer and just tell how to look at the log files. I get that the ISY thinks isn't not getting a response from the module but what do I do about that? It was all working before. I have over 40 working modules. I have a new on/off switch installed right beside an insteon controller that has been there for 3 years. It doesn't allow the new switch to be added. I just updated the ISY to 5.0.3 last night hoping that might fix it but still same problem. I don't know what else to do besides junk the whole system. Please help! Anyone?