As long as the housing is not (or becoming) cocked/offset to one side, usually it'd go down with just a bit more than medium effort of your hand power. The O-rings and contact points of both housings need to be well oiled using GL-5 gear oil. If it still won't go down then some light tapping should get it to go as long as you're sure nothing else seems to be holding it up.
I'd also fear that quite possibly the inside diameter of the bearing carrier new seals that touch the outer prop shaft could be creating some friction should you not be using the seal protection sleeve tool which is available here at iboats.
http://www.iboats.com/Boat-Parts-Ac...07--searched.1--**********.229584460?q=884807
Hopefully you're using a Volvo service manual to help guide you along.
Just to be sure, is the Volvo carrier part number you bought 3851234,
http://www.ebay.com/sch/i.html?_fro....l1313.TR0.TRC0.H0.TRS0&_nkw=3851234&_sacat=0
The image of the seal tool may not show below, but I'll give it a try anyways,