My D.G window wont close, it seems to have dropped, how do i adjust it? ?

I had double glazed windows fitted last year and recently one of them started to drop and now wont close unless its pulled really hard. Im wondering if there is a way to adjust it without me having to call someone out. There is metal scissor action mechanism at the top and bottom of the window (side opening), there is purple plastic things that slide, ive had a go at moving these they dont seem to do anything. Does anyone have any ideas how i can do it myself or is it best to call someone out??