Curtains Sticking On Curtain Rail

Ewescool

I've got those sort of curtain rails where a strip of plastic poppers sewn on to the top of a curtain fits into a slot on the curtain rail to be drawn across the window.

The curtains on some of the windows in my caravan are sticking quite badly and I'm not sure what to use to make them slide more easily. Obviously, I don't want to use anything that may mark the curtains.

Has anyone any suggestions, please?