Here's a few things to consider that may help you:

Since you aren't cooling you're going to have to stay on top of palpating the girls to know when they're ready for the male. You can't just throw him in any time you want and expect a successful breeding.

Adding a shed skin from another male may encourage your desert to breed. It's a trick I've used a few times with reluctant males and it usually helps. You could also try actually adding another male in there with the desert and the girl, keep a close eye on them for a few, then remove the second male. The second method is a little more stressful, especially for the male you aren't keeping in there to breed, but I've done it more than once with excellent results.

If I think of anything else I'll return and post, but as long as your husbandry is spot on and the male is mature it's most likely an issue of timing.