You have moisture behind the caulk this is a mold heaven rip it out and next time do not install a soap dish into the shower corner every time i go to a clients that has problems in the shower it almost always is the niche or soap dish that started the problem. Do large format smooth tile and use suction cup soap dish or get an all in one tub and shower acrylic with ledges built-in.

Time for a gut.

Ladies, of course you are both right. Why is it so difficult to convince the man of the household that it's time for a new shower?? How many times will he have to recaulk this thing before he breaks down?? Arrggh! Thanks for your input :)

20 years...enough said!

Have a mold abatement company out to test maybe that will convince him this stuff will make you sick

Check the 'Use by____" date on the caulk. If it's too old, it won't form a proper skin.

After you run the line of caulk out of the tube are you going over it with your finger to "push" & smooth it? Looking at the pic I get the impression that you're running the caulk out of the tube and that's it, so it's just sitting on the tile surface & with no pressure being applied it's not adhering to the tile.
When you run your finger along to smooth & seal put some detergent on your finger first, so that it slips smoothly along the caulk. Before caulking wipe the area with methylated spirits or isopropyl alcohol to remove any soap/grease residue. If you're already doing this then please disregard my advice.

Howtogirl, yes we even used a special tool for pressing and smoothing the caulk into the seam. What you're seeing in the photo is how the caulk has lifted away from the seam. I think the others may be correct. There's moisture and possibly mould, despite our many attempts to kill any mould and dry out the space, that is causing the caulk not to adhere.

Maybe there's residue left from the original sealant used? If there's even a trace of silicone film left on the joints, caulking isn't going to do well. Even silicone doesn't like to stick to silicone, none the less an acrylic latex...