Is it the old-style aluminum tiller extension? If so, put it under water and work the entire thing throughout it's range and keep locking and unlocking it. It will probably work better.

As far as it coming out, I don't recall what keeps that from happening but I seem to remember it happening to me every now and then too. You may be able to carefully squeeze the end of the outer sleeve to retain the inner part.

Or, then again, you can go buy an $18 aluminum painting extension stick and use it instead.