Every single playtest that is run with PlaytestCloud incurs player drop-outs (when players begin a playtest but do not finish all of the tasks correctly before the playtest expires). This is the reason we always send your games to more players than you ordered - so we can deliver the requested amount of videos within a short window of time. Typically game studios don't see this end of the process - apart from one exception: longitudinals.
When one runs a longitudinal study, results are published on a daily basis after day 2 of the gameplay. But, if you have more than two days of gameplay, almost always at least one player will not complete all of the tasks correctly before the playtest expires. Players drop-out of the study for many reasons: perhaps they become busy, forget to run their sessions, or perhaps a bug in your game dissuaded them from continuing.
But all is not lost. If a player does not complete all of the sessions, that player will be replaced with a new player who did complete the sessions in their entirety. As a bonus, you will be able to keep the data of the un-finished player as well as the new player.