I tried 2 different recipes (both had powdered sugar, glucose, glycerine, water, shorthening. Only one had gelatin (Colette Peter's recipe). Both felt fine when I made them. They were wrapped tightly and placed in tupperware.
When I tried to use them the next day they were brittle and cracked easily. Could not cover a cake to save my life. Tried adding more shorthening - no good.
Any suggestions?

Try placing some of it in the microwave for 10 second increments and knead it in between to see if it softens up.

I've had luck doing that.

I usually use a marshmellow based recipe for my fondant and then when I am working it, I place gloves on and spray them down with pam. This keeps it from sticking to me or my counter when I working it, gives it a nice shine if you need a highgloss finish and keeps it from cracking on me. I usually do this technique if I am making cakes for my kids at school and have to make them a couple of days in advance.

add glycerine, that will help

I have had this happen so out of frustration I doused it with water since I figured "heck if it's dry then lets see what happens when I over saturate it with water". I figured I was going to trash it anyway so why not experiment a bit. Well, it gets all sticky and gummy and I was sure I ruined it but kept on kneading and kneading using shortening on my sticky hands and eventually it reconstituted and became usable fondant and was even better after resting for a day or so. It's a worth a try if you are just going to toss it anyway. If you do it though just know you will be certain it's a gooey hopeless mess before it starts to come together and be usable. Lots of shortening on your hands.

I should say I've done that with the homemade fondant recipe with NO glycerin. Just MM, PS Sugar and shortening. hth!!

It could be that you added too much powdered sugar. When I first made fondant from scratch I put way to much po.sugar and the next day it was impossible to work with. It also depends on the environment that you live, I just recently moved to a more humid climate and have noticed that I have to add more po. sugar to my recipe than when I lived in a drier climate. Also try kneading in some shortening to see if that would help some.

Thanks. Is marshmallow fondant good for covering a cake? I thought it was used for draping and bows.