I had that problem and started soaking them in isopropyl. Worked like a charm, even on the attys I thought were deader than mackerels.
Look for the
thread in this folder on cleaning atomizers--Tony posted a link to a video.
Short version: soak atty for a few minutes (fifteen or so), then dip it into the alcohol and let it run
through (out the battery end). At first it will dribble, then run in a nice steady stream. Then wipe the atty off and let it air dry. After it is dry, prime and
vape.
I keep the alcohol in a tiny plastic container with a snap-on lid and rotate between two attys--one is drying while I am using the other. The atty gets cleaned daily. This has been working extremely well for me.