The single goalscorer prediction ran into complications on previous PLs when a poster picked a player who happened to be part of a game that got postponed at the last minute and they didn't have time to make a change. I used to allow them to make 2 picks the next week to make it fair but that was just adding even more complications so we dropped it eventually.
Yes I recall this now you've mentioned it. One way to resolve this is for everyone to keep an eye on games and if they see their chosen game is postponed, then they will still have time to choose another player before kick off. I will also bump up and mention this should a game be called off.
Once again I'm just trying to bring in more incentives, any ideas yourself?