there are a couple of things that could go wrong
1 o rings (losing/misplacing one in cleaning ...) imprtant ones to check are the one on the chimney where the coil connects to the top and the one on the bottom of the coil
2 the silicone disc on the topfill
if that one dousnt close decently air gets in liquid goos out
check it if it looks damaged replace, if the little lip sticking up is damaged apparently you can cut that off with a sharp knife and it should work again to (its on youtube somewhere)
3 especially for the first assembly, withouth lubrication (some ejuice) the treading/ base and top orings can be stiff as hell making for a bad assembly
my first assembly leaked 2, i had the topfil open once in pocket guess the mess, and i had a wicking issue on the rba with some leaking
for the rest it has been rock solid for me on both the tripple as the quad cores and the rba