Did you buy this particular SA Bren direct from Historic Arms, or was it purchased through a 3rd party ?
Reason I ask is that some of the early SA Brens had similar problems with light firing and extraction.
I bought one of the first ones made ( #0023) to be exact, and had nothing but problems. I sent it back to Len , who ended up scrapping it and building another from scratch which worked fine.
One thing to be aware of is using old milsurp ammo. I used 70's HXP Greek surplus which ran like a top. When I tried to use old POF and 50's dated cordite stuff we had nothing but trouble. Hangfires will also cause major problems as well.
Len also built me an L4A3 based on a Mk II Bren conversion, which is the sweetest and most reliable Bren SA I have ever owned.
Bottom line, if you're having trouble, send me a PM and I may be able to help you with some advise.
Gary