Silicone caulk isn't good for life. The chemicals and materials begin to deteriorate over time, reducing its effectiveness. One sure way to know that your caulk is too old is if it doesn't cure. Silicone caulk past its shelf life will maintain the sticky, moist consistency even after the recommended curing time (generally 24 to 48 hours). If your silicone caulk is more than a year old, get new material. If you think it may still be good, apply a little caulk to a non-porous surface and let it sit for the recommended drying time. If it doesn't cure there, it won't cure on your wall either.
Silicone caulk contains its own curing agent that encourages it to dry on the surface. For this to activate, this agent requires the area around it to be full of moisture. If you live in an arid, dry area, you'll notice that the silicone caulk will take a really long time to dry or won't dry at all. If you're worried about the caulk not having enough moisture to dry, consider putting a bowl or bucket of water in the room to increase the humidity slightly. Don't run any dehumidifiers or heaters nearby, as these will draw moisture from the air and make it even harder for the silicone to dry.
Proper surface preparation is the key to many successful home repair projects, including caulking. You must remove all old caulk, as well as any surface contaminants from the area where you're applying silicone caulk. Contaminants such as mold or mildew, dirt, soap scum, grease and old caulk will absorb into the caulk and can change the way the chemicals, including the curing agent, work. This can lead to an extended cure time. Scrape out all old caulk and clean the area with a solvent cleaner, such as acetone or mineral spirits, before applying the new silicone.
Silicone caulk can also prevent a number of problems for you. If you get it on your skin or clothes, be prepared for some serious scrubbing, as the material will stain. Also, the curing agent in the silicone gives off acetic acid, which you are most familiar with from the smell and taste of vinegar. These acid fumes will irritate eyes, nose and throat, so open a window or run an exhaust fan to avoid irritation.