Sewers in play and active; trash a card with Priest, receive +2, subsequent trashing of next card with Sewers should yield another +2 (because of Priest), it does not.
That interaction is correct as you described it, but highly counterintuitive; you are not the first to get it wrong (that includes me). Take a look at my explanation here in this forum (http://forum.shuffleit.nl/index.php?msg=14618;topic=3318.0#post_PandS). If it's still unclear what happens after reading this, ask again.
Very interesting, thank you for the reply and for the link to a solid explanation.