Are the channels in the inside of the frame of UPVC windows required?

Unscrew the hinges from the frame, then slacken them off on the sash & move the hinge down this will raise the sash when you put it back in. There's adjustment slots on the hinge & at least one fixing screw. Lee

First ,with the window closed,from the inside, look to the top corners of the opener,if there is a gap between the opener and the frame it indicates that the hinges (friction stays) are worn and will need replacing to cure the problem.cutting slots will probably only work for a short time till the hinges pack up all together. Assuming the hinges are sealing the opener as they should ,open the window and make sure that all the screws holding the hinges are tight . If they are loose find some fatter (NOT longer) screws to replace them. What you are trying to achieve is to raise the opener UP so it clears the frame at the bottom. If the window is on the first floor you will need a ladder. Remember, unless you remove the double glazed unit, the opener is going to be heavy,so if you attempt to remove it glazed it will be a 2 man job .