no I haven’t had trouble with that yet. Usually I let them tell me what they wanted and then what they estimate how long it should be. at that point I can tell them whether that is do-able or not.
if you feel like you have wrought everything from the prompt that you can, I’d just call it quits and be fair to the person commissioning you. no need to try and get the word count up for the moneyz if the content is just drivel; likewise, you can’t just suck the good stuff out of your fingers simply because they want it to be a certain length.
Either they have to give you a bit more to the prompt at this point/tell you what scenes they want in there, or you’ll just have to call it at that point