First, I assume you mean the sporting rifle, not the light machinegun.
When that happens to any gas operated rifle, the two things to check first are the gas system and the ammunition.
I am also going to assume that the rifle worked OK for the gunsmith, so ask him how many rounds he fired and what ammo he was using. Was it different from what you are using?
If the ammo is OK so far as functioning the rifle goes, could it be fouling the gas system? That is rare with modern factory ammo, but could be a problem with some milsurp or handloads.
Lastly, when you got the rifle back, did you try it immediately, or did you take it apart? That is not a complicated rifle, but it does have to be assembled right.
Jim