Silicone caulk can go bad, despite the fact that it is extremely durable once applied. Check the expiration date. The shelf life of silicone caulk is shorter than that of other caulks. It might harden in an unopened tube if it's more than 1 year old.
Test Before Using
Mark the date of purchase on the tube. Before caulking a joint, squirt a small amount out of the tube. It should smell like vinegar. Make sure it develops a skin and hardens into a rubbery texture within 15 minutes.
Do not apply new silicone caulk over old silicone caulk. The new application won't adhere well to the old caulk, and your joint will leak. Clean out all the old stuff and dry the area before you reseal the joint.
After you apply silicone caulk, give it time to dry. Use a fan if necessary. Don't try to paint the caulk, because it would probably peel.
Once you've opened a new tube, use it within a few months. Otherwise it will harden and become useless.