I don't have experience with Castle Malt, but from my experience with Weyermann and Steinbach:
Assuming you're in the US, the malt is probably older and not as intense as when fresh. Even when fresh Weyermann is not very pungent (and it seems to be decreasing further) and from the replies in this thread neither is Castle. I've tried <10% in dark beer before and it gets you complexity, but not much of smoke. In fact I couldn't pick it out with all the other stuff. Only that it tastes richer compared to when it is not present. Depending on how much smoke you want, don't be afraid to substitute half/all of your base malt for smoked malt.
The thing with smoke and freshness is that you need a lot of trial and error to dial in the right amount, but compensating for freshness makes that extremely difficult sometimes. I usually err on the high side and if needed I'll age the beer very long, although admittedly that almost never happens.