Why is my shower curb failing?

There are a myriad of reasons that your shower curb is failing, mainly improper installation.  Below are just a few ways that shower curbs are built incorrectly.

1.  When the curb was installed they did not slope it properly to allow water to run off back into the shower. Examples of improper sloping would be, no slope at all (flat) or worse sloping towards the outside of the shower, allowing water to escape.
2.  Building the shower curb using floor or wall tiles which require grout joints.  Grout joints are the weakest part of the shower curb because when they crack and break they allow water to penetrate through causing the thinset (this is what bonds the tile to the substrate) to "wash away" loosening the tiles.  Water damage can also "swell" the substrate, depending on what material was used, and allow for mold growth.

The repair

The remedy for an improper curb installation requires the rebuilding of the curb.  This means removing the shower frame / glass, removing the tiles from the top of the curb and potentially the inner and outer tiles from the rise of the curb, repairing any water damaged substrate and then re-installing the tiles a the interior and exterior rise of the curb and then installing the new single piece curb/threshold, grouting and finally re-installing the shower frame.  Typically this process takes 1 - 2 days for completion.