Yeah, we get a lot of that around here, and this is chess.com's stock answer:
It's actually pretty accurate although they don't mention temporary disconnects which seem to be the source of many of the problems. When your opponent is taking a suspiciously long time to move, it's a good idea to refresh the page, just in case he moved and you didn't get it. And yes, WIFI connections are less stable than ethernet. and here's the article:
I was playing a game where I was ahead. The other player didn't make a move for about 5 minutes, then I said white won (opponent) due to inactivity. I was the waiting for them to move. What gives?
Screen grab seen here http://imgur.com/zDRYfQw