I realize of course that other ponies may have been the ones to tell a 'little white lie' first, and you sometimes have difficulty differentiating between an actual problem and just a polite excuse, so perhaps I might offer a wayfaring rule: Ask once, if they decline for any given reason, express your wishes that they could attend, and ask if they might be interested some other time. If they express genuine regret that they'll have to miss or can't attend, then you may offer to reschedule or help them reschedule I suppose. If they simply express that it's unfortunate and that 'maybe next time' it may be better to assume that they aren't interested in that specific thing.