Smog Test Mystery

I have been taking our 2004 Tioga 26Q to the same smog test only shop for a number of years. Each time the Asian technician tells me that it did not pass the test, but that he would get it to pass, (and he does it). I am the original owner and the engine/smog equipment has not been modified. I recently had the engine properly tuned up by a respected garage. I ask him what causes the test failure. He mumbles in a thick accent that the engine might not have been warmed up enough on the way to the shop, or that the catalytic converter may not be working properly.( no specifics) BTW, the rig has only some 11 thousand original miles

I wonder if others have had similar experiences with smog testing on the same E-450 engine.