Is this cob oven indoors or out? What is the climate like?
It sounds like the inner layer is what I have read about as a sand-rich "thermal cob" layer, and if formulas were followed blindly with inferior clay to start, there might not be enough clay in the mix to stand up to use. There probably is no reasonable way to refurbish the inside of this oven, as thermal cob has no fiber (which would make it lighter and more insulating as well as stronger).
The only possible but likely not practical way I can think of would be to cut a hole in the top of the oven so you could rebuild a core form (a mound with the inner space shape you want) and pack new thermal cob tightly inside between form and existing outer cob layers. I wouldn't recommend this but it might work.
Cob is completely recyclable, so you can demolish the failing parts of the oven and rebuild the dome with only labor and little new material required. I recommend Kiko Denzer's book
Build Your Own Earth Oven.