Silicone should not degrade unless strong soaps are used or mold is
impeding the caulks bonding. Try scraping out the ould caulk with a
razor blade , being sure to be to clean granite , clean with a new
scouring pad to remove any soap residue and clean the area with bleach
to remove any mold. Old silicine caulk will never bond. Get a new tube
from a store that sells alot of caulk and check the expiration date. Ive
had old silicone tubes that never dry or fail to bond. Shelf life can be
1 yr on some caulk. You could talk to DOW that manufactures most caulks
and tile stores for the right product. Automotive caulks may be
tougher. On my granite regular colored caulk was used and Ive had no
degradation. I would suspect the caulk or surface prep, contaminents,
mold- soap scum.