I'll echo a lot of comments that your problem is likely because it was on a telephone pole...they don't have any bark. Climber stands usually grip best when the tree has some bark that the teeth on the stand can dig into. Bare wood (like a pole) or smooth bark, like on some hardwoods or aspen, are prone to slipping, I've found. I like older trees with rougher bark with some give to it, because the teeth on the climber bite better. What I do is climb to my desired height, and then bounce lightly on the platform to dig the teeth into the bark. Then I tighten the security strap around the tree as best as I can to prevent the stand from slipping if I adjust my weight on the platform or seat.

Instead of practicing on a pole, try it out on the kind of trees you expect to hunt from. You'll find that each type of tree is different and presents a different challenge. Old Red Maples, for example, have really rough bark that comes apart in plates, so your cable will often catch on them and noisily rip off chunks of bark as you climb if you're not paying attention.

Also, +1 to the comments about angling the stand upward when you start. It takes some practice to figure how much lean you need to start off with depending on the taper of the tree and the height at which you want to hunt.