Firstly I would get hold of a Torch and have a really good look at the Grout, look for hairline cracks, if there are any then it may be that there is some movement in your shower and a flexible grout should have been used.
Before you go to the time and expeanse of ripping out the old grout I would go over the Hairline cracks with a fresh grout < Flexible > Try Mapei Ultra-colour for this, see link below
If the problem persists then it is time to rip out the grout and start again, hopefully this wont need to happen.
Regards Russell at Tile Doctor
Tile Doctor Ltd