I would talk to the CFI, and see if he can get one of the Robins out of that field and into the field where he teaches on his "off" days. You can then do your skills test with your CFI from that other field.
Furthermore, let the CFI know that you would be available for a skills test anytime, with about two hours notice. So if he's got a cancelled lesson he can phone you up and you can do the skills test right there and then.