Gap in middle but ok at top & bottom

At the hinge side of my double glazed window, there’s a gap between the seal and the window casing, letting in a draft. It’s fine at the top and at the bottom. I can see outside through the gap so it’s not sealed, either internally or externally. The gap tapers off to a tight seal both up and down towards the two hinged corners. Any ideas how to fix please?

Wedge the frame out

sounds like the window frame has been overtightened to the wall, and pulled over , as Woody suggests , unscrew and see if you can get a wedge in there. However a picture posted up might show the isuue is something different, Pictures from inside and out if poss, need context of positioning so not too close up!