In the scenario of a non-adjustable gas block, the only issue for an undergassed gun is to drill up the gas port.
As long as you're sure the adjustable gas block isn't the issue, might as well yank it off and measure the gas port diameter in the barrel. Even a wide-open adjustable gas block isn't gonna deliver enough gas pressure, if that gas port is undersized.
Report the result of the gas port size in this thread - we have a pretty good idea is what you'll need, diameter-wise, and we'll certainly recognize a too-small port diameter.