Help with toilet seat fixing?

We have a new Vitra soft close seat which keeps working loose. Usually after 4-5 days one of the hinges goes wobbly and we have to start again. We have had the seat off about 5/6 times and have been meticulous in following the instructions, measuring carefully to get the hinges even etc. Also when opening and shutting there is a little scrunching noise which sounds as though the screw fitting retaining lugs are rubbing at the porcelain underneath the hinges.
Any help much appreciated.

Pat
May 2009