Maybe you need to get into boost more ;-)
Seriously though. Maybe the vacuum line needs to be smaller as it's not getting enough vacuum? Maybe too much vacuum line is creating issues and you're losing efficiency of flow? My Arrington can is huge, I don't recall my hose diameter but it's working overall. Think like a Shopvac...I'd imagine it's better to go smaller diameter hose vs larger so it'll suck better.
I've mentioned this before, but there are two systems available for the N54...BMS makes a setup that only works under boost, then RB makes a setup that works out of boost. I'm not smart enough to figure it out, but we need to find a way that'll cover us on both sides because I have to empty the GF's 535xiT every month vs mine that I could probably get away with doing it every three months.
http://www.burgertuning.com/BMW_335_oil_catch_can.html
http://www.rbturbo.com/rb-n54-external-pcv-kit