2 things to look at on the 390.
1. as previously stated look at the gas ports, and piston area. Make sure the area is clean.
2. (This is what got me!) The recoil spring on the 390's is bad about rusting! yes, rusting. This was a serious design issue, IMNSHO! The spring piston, and receiver extention need to be replaced with a stainless steel unit. (Brownell's has it for ~$125). Otherwise you can disassemble the unit, polish everything out with a dremel tool. Hone the extention out with a homemade hone. Soak everything in like 90wt gear oil, and reassemble.
If you don't feel comfortable doing this drop me a line at
RotDorn@midsouth.rr.com I live in Cordova, and will be happy to help you fix this issue if you need it.