UPVC windows not sealing tight when closed

Our new house has a couple of windows less than 10 years old but they do not seal properly when closed. They are tight on the latch side but loose on the hinge side, which means they're draughty, howl in winds and don't cut out general noise. Is this an easy fix? I assume it should be covered under 10 year guarantee with the window company - we've called them about it before but they have ignored the units and just re done the mastic around the frames.