I've found it can be intermittent too, not sure how much tempreture is a factor and also length of time the boot has been unopened. These do seem to have an impact in my experience. It clears 99 times out of a hundred now, and so far I have:
Replaced pop springs with genuine Zmanalex/Nissan pop springs
Replaced boot struts (this was to stop tailgate falling on my head when open, not to assist boot pop!)
Removed rear wiper (this was for cosmetic purposes, but there is a not insignificant weight reduction too)
Removed boot weight
Added rubber "bumper" feet (approx 1cm in height) to the pop springs
The last two on the list had the most noticeable effect, but were also the last steps I took - so not exactly scientific proof
I don't experience any drafts or any increase in noise since these have been done, there's no water ingress (which would suggest the seal is still tight).
Anyway, that's all the steps I could find on here and it's now sorted, fingers crossed. Glad you seem to have it fixed too bud, hopefully it won't come back.