I had same problem. I roughed up the surface of the pole by scratching it with a knife and stuck the clamp on with "Serious Glue" £4.99 or similar from B&Q, it has lasted a week so far.
I did tell Gardiners and they sent me some 2 part araldite type glue straight away, but it needs a small mastic type gun to use it and "Serious Glue" is just so easy.