First, I have never use the caulk strips, caulk in a tube or any of that stuff. This should not make any difference though.
Sounds like something is on the surfaces to be caulked that is not allowing the caulk to stick. I would scrub the areas with bleach and water and then make sure that all the soap scum is off completely.
Make sure the area again is dry.
Keep the door open during drying time to allow fresh non humid air to circulate over the caulk.
I have had only good luck with caulk in caulking tube, or to be used with a caulking gun.
Get a good quality of 100% silicon caulk from DAP, it will cost about $5 a tube. One should do it.
Apply your bead all along the corners. Then with a wet finger or a finger dipped in rubbing alcohol rub along the caulk to make a nice ramp and to smooth it out along the edges of the wall and floor. Do this all around.
One day should do the trick.
you might be using a caulk with all or too much latex in it. Go 100% silicon.
Now, something else to think about. Very harsh chemicals that you use to clean the shower stall or walls might be compromising the caulk also. I know that bleach is hard on caulk and makes it eventually break down. Watch this in the future.