Ok, I decided to build an AR. First trip out was yesterday morning to dial in scope and adjust gas block. Once I found the sweet spot with the gas block it would run through a magazine with no issues. Then I took a break, 4-5 minutes, when I go to shoot again there's not enough gas (ejecting but not stripping off the next round). Off with the handguard and go through the process again - shoots great until I set it down for a few minutes. When I go back to it - the same damn thing, not enough gas. I turned the gas up, down, every which way and it always did the same thing. What am I doing wrong?