Double glazing - espag problem

I have a problem with a side window. I cannot open the window. It appears that, when unlocked (i.e. the window handle is put to the open position) the top espag claw unlocks (the top corner of the window can be pushed away slightly) but the bottom espag claw remains closed. As there is a bit of movement in the centre, it appears that he centre bolt is unlocking OK.
Any suggestions?
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