Mosiac failing after a certain point

Has anyone had timing issues with mosaic mode? Or can it only run for so long? I tried twice last night, same target, absolutely clear skies (a rarity) and after a little less than 2 hours, i got constant fails even tho the target was visible and below 80 degrees (and it hadn't quite filled in the whole target area). I stopped it, went to another target, then went back to the original and then it was tracking fine again for about 2 hours until constant fails (but this time it did fill in the whole target). Does it intentionally stop after filling in the whole target, or should it continue to stack to help SNR? Any thoughts on this appreciated