Truth be told it can be very frustrating. Sometimes you put a lot of effort into coming up with interesting topics and ya get zilch. Can happen multiple times in a row. I’d even been vexed to asking what the deal was directly a few times in the past when it was really bad for stretches. If people didn’t like the challenges or the time slots we were running, et cetera.
Really there’s not a lot to be done about it. You just have to realize and accept that people may have other things to do at that time- particularly if they can be paid for their art by commissioners. They’ve got their own mood, energy, interest in the subject…
Be patient, encouraging, and don’t be too complex. Creative challenges are great, but simple works well too. There’s really no telling what will work so just be persistent and positive,