I’m having a similar problem where the tiles meet the bathtub. The problem is that this area cannot be grouted because the weight of the tub changes and moves whenever it is filled and emptied. I tried filling the tub before caulking but like yours, my caulking is still tacky. I’m wondering if the backerboard might be wet ,from an earlier problem, and is preventing the caulking from drying correctly. I now have the bottom row of tiles removed and found that the backerboard looks wet but is not drying after about 2 weeks. I will have to give in soon and chance reglung and recaulking onto what appears to be a wet backerboard. Good luck , I’ll be watching for any other suggestions as well.