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Internal doors not opening smoothly / get stuck on carpet

Hello, We have recently moved into a place that has been newly carpeted and the doors sticking are now just getting to me. There are up to five internal (including two walldrobe) doors that are hard to open due to the new carpet; and potential a slightly sloped floor. I believe the doors are hollow (except the walldrobe doors which seem a bit more solid). I have also checked the screws in the hinges to see if they were loose, I do not believe that this is the problem. The doors might just need a little sand / plane. Could you please give me an estimate on how long, and how much this might cost. In terms of urgency, there is no rush on this as such, so am flexible on this front. If you have any questions, let me know. Cheers,