This is a very common problem (I have just had it happen again) as the "tooth" mechanism inside is made of aluminium and breaks quite easily.
You can either buy a new/used arm or if you are brave take the arm appart and using replacement parts sold on eBay (in NL) fix the problem ...
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=7954534976
I have ordered the parts (they seem expensive but compared to a new arm ..) and should have them soon, will post on my results.
You can either buy a new/used arm or if you are brave take the arm appart and using replacement parts sold on eBay (in NL) fix the problem ...
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=7954534976
I have ordered the parts (they seem expensive but compared to a new arm ..) and should have them soon, will post on my results.