It could be compression wicking then ( when the nappy gets squeezed and wee comes back out) . Ways to help would be to change more frequently but that might only give you an hour per nappy if you've got a mega weeer
12 weeks is a point at which they all wee a lot too due to the amount of milk they have at that age. It may be when they get bigger it won't be a problem. Babies wee less when they are on solids.
It could always be a fit issue too.
Lots of people do abandon the popper vest, this is also because people use wool covers or trousers. I used vest extenders so the vest was done up but not squeezing the nappy.
I only found sucess with a two part system; a nappy then wrap. My cheap as chips prefold and nappy combo was my turn to nappy if I wanted to guarantee no leaks.
Microfibre is great for being slim and soaking up quick flowing wee quickly but once saturated it causes problems. Cotton or other materials like bamboo, hemp, zorb hold more for longer.
I hope that all helps, so sorry your first go is not proving successful