No other waves coming from the abyss pits after the first wave is destroyed

Meaning no more rare/normal abyss monsters spawn in, leaving the abyss unfinished. Only time this has happened was now on a tier 15 map "Creek" with the modifier "Abyss pits in area are twice as likely to have rewards" - I've only just started running this tablet modifier so maybe it's part of the problem. It still looks like the area near the abyss abyss is occupied on the minimap (A few dots near abyss pits look green even though no enemies are nearby - also checked whether they got stuck on terrain and couldn't find anyone) The other dots are white though as they should be since no enemies nearby. Just 1-2 dots near abysses are green.
Another abyss modifier on the map at the same time was "area contains an additional abyss"