Fixing broken uPVC window mechanism

I have a uPVC window which won't open anymore. The handle turns ok and I can tell the top and middle catches release but not the bottom one.
I have removed the handle and turned the mechanism manually with the same result. The window is not very old.
Does anyone give me a clue on how I can fix this?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.