Your complaint is among the most common I see in the forums. This site is huge and bad things happen when it runs out of memory; usually a temporary disconnect that you don't notice. If your opponent seems to be taking too much time, refreshing the page might reveal that he has indeed moved. There are a bunch of things you can do though to minimize the chance of a disconnect and a number of articles in the Help section. Let me go get you one...
https://support.chess.com/article/213-how-do-i-fix-my-disconnect-lag-issues
Hello,
Today I played a rated rapid game (https://www.chess.com/game/live/6006181033) and on half of it, I made a move (specifically f6). After waiting some time for my opponent to make a move suddenly game abandoned window appears and tells me I lost due to abandonment, losing some rating points. After I closed that window I realized my last move was reset so probably something didn't register correctly. I can assure you the internet connection was excellent (full bars) and I had no reason not to move as the game was pretty much equal at this point.
Thanks in advance