Warped window frames

If the frames are warped they will need fully replacing and I'd get a surveyor in to find out why they warped 15year old windows shouldn't do that

They will need to be replaced as it is not possible to remove bow in sashes. They are old and probably have insufficient reinforcing. Hope this helps

I would say that the windows were poorly installed in the first place, yes, I would recommend new windows as they’re probably distorted now

Are they timber or PVC windows. PVC can be adjusted to some amount easily to close up gaps there are also parts that can be added called pull in blocks. The frames can also be refixed to be more in line. You need a good glazier and they should be able to look and tell you exactly.

I would agree with "Edwood" there is some degree of workability with plastic windows, not necessarily a bad installation, maybe a bad product. There are quite a few variables to take into consideration, orientation of the house, ie south facing, are the windows a foil product? darker colour frames tend to have more movement in the sun, maybe attach some photos for us to see? could be able to give a more accurate answer with some accompanying visuals.