The few times I couldn't do it I took it as a day that they didn't quite do as well... because it's so rare that it happens. I know with the can't make the serum one I racked my brain then just bought the seed packs.. think I needed spinach that time. (Right now plants can't be harvested at the science lab for me .. some kind of bug so I'm bringing the plants from home). Otherwise it mostly asks me for things I can do and do easily. Sometimes I don't want to do what it asks though.. so I try everything else first and hope the new ones that appear I want to do more.