The reason: new silicone over old silicone usually fails because the new bead is bonding to contaminated material, not to the actual tile, glass, bath, benchtop, or substrate.
The real bond is underneath
Sealant needs clean bonding faces. In a bathroom or kitchen, old silicone can carry soap residue, mould, body oils, cleaning product residue, and moisture. If new silicone is applied over that, it may look neat for a short time, but it has not solved the failed bond underneath.
Why it peels
When the lower bead is already loose, the new bead moves with it. The fresh silicone might stick to parts of the old material, but the old material is no longer stuck properly to the joint. That is why the patch can peel as one piece or split along the same line.
The better repair path
For failed bathroom or kitchen silicone, Brandon would normally remove the loose or contaminated bead, clean the bonding faces, assess whether the joint is dry enough, and then re-seal with a suitable wet-area silicone.
When topping up might be acceptable
There are limited cases where a tiny cosmetic touch-up can make sense, usually on fresh work where the original bead is clean and bonded. That is different from covering failed, mouldy, or loose silicone.
Why this matters in rentals and handovers
A top-up can create repeat callouts. If the area is a rental, sale prep, defect list, or builder handover, it is usually better to fix the failed joint properly once than make it look acceptable for a few weeks.