The top cap design is complex compared to most. You have to turn only the round section to open/close the tank, while leaving the triangle section alone. When closing the tank, I found it's easiest to turn the top cap to the left until you feel the threads catch. After that it's much easier to screw the top cap down. You also have to work against the tension of the oring in the topcap.

I own two Ulton clones of this atomizer. Both needed a 'break in' period. The top cap and JFC would occasionally stick when they were new. It eventually smoothed itself out after a few tanks full. Well, it's either that or I perfected the process I described with the top cap above lol

Either way, this thing's top cap just takes awhile to become accustomed to using. It's def worth giving another shot if you already have it sitting around. And if something with yours is defective, now's a great time to try another since they're less than $11 :)