> I can't figure out the reason X doesn't work
This is more often what I really mean, i.e there is some obvious solution (to me at least), but it's too obvious that they have likely already considered it and there is an issue with it... but they have been working on the problem longer than me - so tell me. Occasionally the solution is obscure enough or slightly unfamiliar to them that it slipped their mind - in which case it's an "oh of course" moment, which also causes no hurt feelings since you aren't trying to rub it in.