I only had that once with a mini Nautilus/BVC coils - I went in search of info and found that if the base is screwed on too tight after filling, it can cause problems. It does seem to become a little tighter in use ..perhaps heat
Try removing the base and then putting it back on until just tightened, then back it off half a turn.
Sometimes, if a fairly new coil seems like hard work, I also try turning the metal top on the tank a little bit to the right and then back again (not too tight) can help -something like releasing an air lock or bubble. Seems to work.
Apart from that I've had very little in the way of problems with them. Always the chance of a duff coil though if all else fails.