Is the gas port clean ? clean with a tooth pick or pipe cleaner
Is the magazine tube clean ( outside) ? Clean with a scotch brite pad and oil and wipe oil off.
Is it just 1 brand or type of ammo or reloads ? try different ammo
Is the gas piston on the right way ?
Has anything changed between when it worked and now ?
Is the O -ring on and in the right spot ?
Are you over oiling it ?
Call customer service and ask for tec help and pick their brains