I've never found them particularly reliable. The last place I tried to use them in, there was packet loss and the ARP broadcasts would keep getting dropped, meaning I would lose connectivity to anything past the homeplug (the router and the rest of the internet) for minutes at a time.
In this new place I've moved into, I tried them again but the speed and overall performance was again pretty awful, so I've opted to link things together using wireless bridging - no issues what so ever.
It could be a fault with the homeplugs, it could be interference on your mains ring, but I have no advice on how you can troubleshoot it. Sometimes, the easiest solution is to just admit defeat, recognise the technology is a fudge, and use an alternative (wireless in my case).