Thank you all for your comments, very helpful and I'm glad it's not just me.
Yes, it's the second pole section from the top - quite consistently getting blocked.
Even filling the section up with water and sloshing it wasn't enough, I had to run a broom handle through it.
I'm just about done, fortunately, but have another place to do on Monday.
Do any of you ever feel that you're getting too old for this...