Doors and Windows - Pella windows are sticking

Our house is 15 years old. We have aluminum clad Pella casement windows installed when the house was built. Over the last several years, some of these windows have become harder and harder to close. The window portion has apparently sagged somewhat and the lower corner now comes in contact and scrapes against the frame. I have checked and all the hardware seems to be screwed tight; nothing is loose and nothing seems to be adjustable. Two of the windows will no longer operate at all; something must be stripped in the cranking mechanism.
I was wondering if anyone else has had this type of problem with their casement windows. Is there some way to adjust or shim these windows so they will not scrape the frame and close properly? Sorry if my message is kinda wordy buy I am trying to describe the problem as accurately as I can. Thank you.

You might want to check for signs of movement in the house......see any sheetrock or foundation cracks that would indicate settling ? It may be pressure on the window frame that is causing the problems. Try measuring the diagonals of the frame to see if it is still square. A casement window has to be "right on" or it will not operate properly. A double hung can be a LOT more out of square and still work.

Assuming the above is not happening, I'd be calling the local Pella dealer for advice.

I've seen this a lot with Pella casements and there is very little you can do. Take Tn...Andy's advice and call Pella.