Architrave window frame uneven

I'm in the last stages of my renovation and about to put the architrave around the doors and windows.
The problem i've noticed is in a few places the plasterboard is not flush with the window frame, in some places it in about 10mm and in some places its out by about 10 mm.
The places it sticks out is not a big issue because I can still attach the architrave to the frame and just pack behind it, its the places where the window frame sits below the plasterboard that has me stumped.

What are my options here?


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