if you have a working repeaters or an observers at loaded chunk and then you unload the chunk and then load it again the observers or the repeater that were powered would stop working. And there is no way to start them again.

here is the sequence to reproduce the issue:

1. put repeater/observer on loaded chunk
2. activate the repeater/observer with a redstone pulse  
3. leave the chunk (that leads to the chunk to became unloaded) 
4. come back to the chunk 
5. the repeater/observer still powered but wont output any redstone signal
6. even if the repeater/observer will be replaced, the replaced unit wont work.