Flagstone patio grout problem

I hope I am in the right forum. I have a large flagstone patio-about 5 years old. The grout between a few of the flagstones seems to be shrinking and cracking. I assume that water will seep in and only make it worse but do not know how to make a repair. Never done this kind of work. Or should I leave it to a "pro?" Any advise would be appreciated.