When the contents of a container changes, the observer does not detect this.
In previous snapshots, the observer detected when an object is placed in a container. But not anymore, for no reason.
In pocket edition, I think observer downright detects the total content change (block addition in a container already filled for example).
PS: Observer detects the "on/off" in furnaces/droppers/dispenser but not items.
Observers only look at block and block state changes.