Hmm.. I've had that.. First, take atty off, take cart out. Try sucking
through just the atty by itself? Any air obstructions? If not ..... try on a few different
batteries, with different cart combo's.. Some
batteries when paired with some atties get hard to draw on, guessing due to threads and openings etc etc.
With the 510 .. those 2 holes at the atty bottom I try to line up with the grooves in the battery. Look close, you'll see them. Hard draw could be due to over tightening the batt and it not being able to draw air through those holes..
If the atty has a hard draw unattached to anything get some canned air and blow it out from end where it connects to battery, make sure you get the holes well.