Brass breech longshot reliability issues

Basically i have nearly finished my BB LS, but i have this odd problem that when there is a dart in the chamber the plunger doesn't peoperly shoot the dart out about 95% of the time... it seems very odd to me, but im just not sure where is the most likely point for it to be leaking the air...
(i have only the stock spring in for the moment.)

Brass breached longshots require more power behind the dart because of the tighter dart to barrel seal created by the brass. More friction + same power spring = the misfires you're experiencing. It should fix itself if you upgrade the spring power.

First of all. Are you using a streamline? Streamlines aren't a good fit for 9/16". If you're using blue foam. I'm unsure.

LIke ApoK says, there probably isn't enough 'spring power' to push the air out quickly.