Usually you can remove any individual training or preferred move training (or first try lowering training intensity if they have individual training), if you want them to quit complaining. Even though coach workload should only effect coaches I've had a couple cases of individuals also upset when my coaching was spread too thin...besides that there's not much you can do.
Maybe those continuously whiny players don't think they get enough rest days (if you want to try micromanaging).
Criticizing their training level might also help with the perpetual whiners, though I haven't really tested that.