I have two theories:
1. They are not ready to deliver. Are they squared off? They should look like this \__/ a few days before they drop fry.
2. They are holding their fry due to stress from your unfavorable stocking. Guppies are tropical fish, meaning they like water around 78F. Axolotls like to have chillers. Plus I think they like to eat fry (not positive). You need to keep your guppies in a separate tank. The females could be so stressed they are holding fry which can lead to problems. I suggest you consider rearranging some things.
Without seeing a picture, theory number two, to me, is the more likely candidate. It could be both, though.