Game stalling

On three occasions in last few weeks, I have been  playing games where I thought my opponent had abandoned. I had made a move and their clock continued to count down. This eventually stopped at but with no indication of my having won on time. On later reviewing the game history, I had “lost”.  When this happened today I was able to have conversation with my opponent. We were both sitting watching a screen waiting for the other to play. I could see his clock counting down whilst he could see my clock counting down. Neither of us could make a move on the board.  I eventually lost on time when his clock stalled at and I apparently still had over 6 minutes. What is happening?