P0101 Mas or Vol airflow A cct range 1.9 tdi

Over a period of time the fuel consumption became worse, the engine stuttered at low revs and sometimes the engine would have to turn over several times before it would start. Thanks to help through this forum I replaced the Camshaft sensor which absolutely solved the starting problem, but the stuttering and increased fuel consumption remained. Last week the engine management light came on and I read the code above. I replaced the MAF sensor and the engine is now back to mid season form, no stuttering and consumption much better, problem solved. However, three days later the engine management light came back on, and the same code was there. I can't find any air leaks, so what might be triggering the fault code?