Yes, I know plastic doors aren't anyone's favourites, Horsey. The thing is though that they are extremely stable, and, when adjusted properly, are draughtproof and rainproof.
Also, as I'm often telling people on here, you can paint them any colour you like. In my own house, all the uPVC windows and doors are currently in Farrow & Ball. Occasionally, people knock on the door wanting to know how it's done ;o) (Just use the right primer)
Anyway, your problem would still be there even if you used better quality flooring.
99% of the time it's simply a matter of adjustment. I had the same problem with my front door. If you can't figure out where all the adjustments can be made, then employ a uPVC fitter to do it for you. Have a look for a small, local installer................ NOT the major chains.