I've been attempting to re-caulk the bathtub in my "new" house for the past couple of weeks. I'm running into a problem, though, with the caulk separating from the tub in places. The only part that needed to be re-caulked is the area between the shower walls (an inserted shower...not tile. Not sure what to call it) and the tub itself. I cannot figure out what is causing the caulk to separate. Below is my process, in detail. Please offer suggestions.

I sc****d off the old caulk with a putty knife and razor knife. There were 3 layers of caulk, the top layer being very hard and brittle. It took just over an hour to get it all off, but I made sure I got it all. I ran the razor knife along the tub and shower area to get any remnants.

I vacuumed the area and allowed it to dry until the next evening, about 20 hours.

I used a caulking gun and caulk for bathrooms and kitchens. The tube of caulk said to let it set for 72 hours before exposing it to water. I waited 82 hours.

Within a couple of days, the caulk had separated from the bathtub in places. So, I stripped it all off again, vacuumed, and let it dry until the next evening. I purchased a different tube of caulk (the kind that doesn't need a gun), and caulked it again. This caulk said to wait 24 hours, so I waited just about 2 days.

Again, it separated. I am now understanding why the previous owners put 3 layers of caulk on the tub. They probably had the same issue.

What could be the problem?