Nat,
I'm assuming that you have already adjusted the nut on the clamp?
Remember it's a left handed thread, so you think you are loosening it to tighten it.
If you've already done this, then you can remove the clamp and file down the two edges that touch together to give you more play.
Other than that it is likely that you need a new top section.
How old is the pole, with constant use I find they only last about 6 months.