What type of caulk was used? Where was the caulk used? If it were not used in the correct location it may never last.

In addition, were the surfaces fully dry before the caulk was applied?

Is the caulk itself cracking? Splitting? Or is it pulling away from and separating from one of the the adjacent surfaces?

Caulk shrinks as is cures. If it's put in too wide of a gap or used to fill a deep gap in a single application it can shrink and crack, or shrink and separate. Same with the gap depth. If the gap is deep, a backer rod should be installed prior to caulking. Backer rod controls the amount of caulk installed in the gap and thus helps control shrinking.

You do have to watch shelf life too. Old caulk, it can crack excessively as it cures or sometimes never quite properly cure.

Then there is structural movement. A bather standing in a shower or a tub loaded with water can cause the tub or shower pan to deflect or move, that repeated movement can cause caulk to fail.

All of the above are good answers for the notion that the application was faulty. When you do determine whether the installation was proper or not: yes - use silicone, IMHO. Hands down. It's a little more difficult to work with but it lasts much longer.

Thank you for your input.
I am not sure what was used at the beginning. It wasn't separating - just cracking.
Not sure if it was fully dry before they applied. I have removed all the caulk that was there and am letting it dry for nearly 1 week. It finally seems dry now and I will apply silicone myself.

Thanks again.

But again where is this being applied? Caulk may not be the correct product to use and it may continue to happen. If caulk should not be used here and the shower it not properly waterproofed you may end up with water issues.

Which silicon do you recommend? I have read that Laticrete Latasil is very good? I am having the same cracking problems with my siliconized acrylic caulk.

I would follow the advice and brands mentioned by bill_vincent on your post as he knows much more about it than I.

Here is the crack in question: