i would guess that when installed the window was not correctly "toe and heeled" this is the method in which packers are used so that the weight of the glass is transfered to the hinge side of the window, quite hard to explain like this but i will have a go. If you take out the plastic beading, (try a putty knife to prize it out) you will be able to tell if this is the problem. there should only be packers under and to the side of the glass at the side closest to the hinge and then the same diagonally across from these at the top of the glass. this steadys the glass in the frame and will make sure the weight of the glass is transfered to the hinge side. You may find that the hinges are damaged and need replacing too or you may be lucky good luck any more questions make another post ill check back soon
yorkshire chippie
October 2009