Hi and welcome to the forum.
To help answer you question, I have a few of my own to help with a response.
How long have you had the lenses?
When the lenses do move what are you doing at the time?
What do your eyes feel like prior to the lenses moving?
Strange questions I know, but if my eyes watertoo much, the lens can float off from where it should be. Rigid Gas Permeable (RGP) contact lenses are meant to move a bit to help tear flow around and under the lens so the corenea does not dry out or suffer oxygen starvation. Gasses move better through things that are damp rather than dry.
During the initial acclimatisation period to the lenses they do move more as you get used to them. In most cases it gets better, but worth keeping a note of so that you can discuss it with the optom at your next visit and they can then check everything lens and eye wise is fine.
Hope this helps.
Regards
Gareth