It sounds to me like it may be time to have a local pro come out and take a look at it. First you say mildew keeps coming back, which sounds like moisture is holding in the shower. Secondly, your question about grouting. You should only have to grout it once-- when first putting the shower together. If you have to grout any more than that, there's movement, or a problem beneath the surface. Either way, the problem (causing the loss of grout) needs to be identified, and alleviated.

Bill V.--I meant to say recaulked instead of grouting. My caulk turns dark in some spots which appear to be mildew. But instead of being able to recaulk those spots I have to remove all of it. I'm not quite sure if you are saying that the caulk should not be mildewing? I would seriously rather hile a professional to do it, but I can't imagine anyone would to come in to do only that. What do you suggest using to get it out, etc.? Thank you for taking the time to answer.

First, I'd scrap all the silicone caulk. You'll beed to hit a hardware store, and pick up a silicone residue remover, otherwise no caulking will stick-- not even more silicone. Once you've cleaned it up, you might want to go to Lowes and take a look at their selection of Laticrete caulks. They're made to match their grout colors, but I'm sure you could find one that comes close to yours (unless yours is white, then it'll be a dead ringer!). The reason I specify the Laticrete caulks is that they have antimicrobial agents in them to help stop the growth of mold and mildew.