Breaking in an engine can be hard on a glow plug. It could be that your plug isn't retaining heat as well as it did at first, and so the engine is getting a bit cold if you try to run it way rich.
Be careful about runnning it too rich. There are several threads on RCU about how to break in a taper-bore engine. The engine needs to get up to operating temp., or con rod damage can take place.