Walls and Ceilings - Ceiling Texturing Question

I am texturing my bedroom ceiling. I've never done this before but it is working out ok so far (lookswise), but the joint compound I'm using seems a little thick. When I purchased it, the guy told me it was ready to use and didn't need to be thinned down, but when I run the trowel over the compound, it tends to want to peel right back off rather than spreading out. My question is--should I add a little water? Thanks in advance for any help.

Maybe a little late here, did you prime the ceiling, was it a new ceiling, was it bare drywall, did it have paint on it already, and if so what type of paint, flat, semi-gloss-gloss? Thinning weakens the texure a little so you want to be careful with it. I used some three months ago right out of the bucket over a freshly primed ceiling and it was fine without thinning.