We've had that happen from time to time with our '04, usually stopping the slide out portion just after beginning and I solve the problem by starting over and just leaning on it a bit and it does fine. If your's isn't even starting to slide when giving it a bit of help I'm not certain what that might be.
Recently we had trouble with one of the locking arms not opening and I've learned not to over tighten them. I was told by my very reliable dealer/service center that the slide will stay in place even without the locks tight and just to make contact with the inner surface when locking. I realize this is not related to your post and offer it as information.
Good luck and sorry not to be of more assistance.
In dog years, I'm dead!