Sounds as though your coils still have residue.
I'm c&p'ing this from an ol' post. It's for top wick clearos, but I think it should work for others:
My cleaning method for the StarDust type clearos (VNs should be easier):
Remove mouth piece/drip tip.
Remove silicone cap and tube with tweezers/needle nose pliers. You will now be able to see your coil.
Flush and rinse with water until the clearo has no remaining joose.
Shake out water and dry the exterior, including the connection end.
Pulse dry burn until you see (1) individual red coil wraps with (2) no 'smoke' or (3) sizzling.
Let head cool.
Replace cap/tube.
Fill with joose.
Replace MP/DT.
Let sit for a few minutes to soak the wick.
Vape your face off!
Be cautious when dry burning. You can burn out the coil if you leave the power on too long. Remember to pulse on, then off for 3-4 seconds each.
Hope this helps ya'll.
Please do note the (1) / (2) / (3) as that's the critical criteria for a clean coil! Good luck!!!