First lift the lid and see what's happening when it sticks open [[flushed position]
1. The syphon wire arm should be straight inline with syphon if not that could be the problem.
Undo the flush rod plastic arm retaining screw and work the arm so it's inline with the syphon and the syphon wire arm is virtical over the syphon re-tighten the arm screw and operate see if handle returns to normal position.
2 if not check the far end of the square flush rod it may be junked up with lime deposits clean that off and prob solved.
nobby
February 2009