My ProTank II cannot get through a full day without eventually gurgling and leaking. It has become a daily ritual for me to remove the head, blow it out, wipe the threads, then tighten it back on. I put a new head on roughly every 5 days.
Unfortunately, yea the protanks and tanks like the iClear 30 while they usually vape really well (initially) do eventually start to leak and gurgle.
Apparently it's all about the o-ring not forming a good seal but here is one video that shows how you might be able to fix it if you have the original box or can go pick up a new o-ring.
I wish I had found this vid before I gave all my protanks away lol.