Modern sealants (caulks) are vastly superior to those available in years past. They exhibit better flexibility and do so for longer periods of time. As good as the newer products are, they still must be properly applied if they’re to perform as intended.
As a general rule, sealant joints should be about half as deep as they are wide. Shallower than that, there’s not enough material to accommodate thermal contraction of the substrates (which widens the joint) without tearing. Deeper than that, the material can be distorted and/or pushed out of the joint when the substrates expand and narrow the joint. Elastomeric sealants also cannot accommodate full movement in two planes and three-point bonding can result in damage and reduced protection. Backer materials (to control joint depth) and bond breakers (to prevent three-point bonding) are critical elements of sealant joints.
Joint preparation is equally important. The joint substrates must be free of contaminants and in some cases, primed. A properly adhered joint will tear (fail cohesively) before it debonds. The accepted test for adhesion is to cut a cured joint across its width and a few inches along the substrates, then pull the freed segment at a 90 degree angle until it tears or pulls free from the substrates. If it pulls free before it tears, adhesion is inadequate.
Other important factors include temperature at the time of application and other weather considerations. Manufacturers will have requirements specific to their products and those must be complied with to assure performance and longevity.
The conditions necessary for sealant performance cannot be attained when applying new sealant over old. “Overshooting” caulk will produce results that are at best, temporarily or partly effective.