Cause it is a staggered roll out for the update the normal way, sometimes the force trick works sometimes it doesn't. I suggest keep trying it eventually you'll get the timing down for the trick to work.
Took me about 50 times but it finally worked. The trick I used was turning off my wi-fi connection before it was even done checking for the update. Try it and eventually it'll work.
But doesnt the HTC Arrive officially have Mango update now? Thats what this says http://www.windowscentral.com/official-sprint-htc-arrive-get-mango-today
