Missing or Broken Aerosol Cap
Cause: "If the cap is broken, it may have been assembled with greater force than necessary," says Garofano. "This can cause the cap to spread slightly, making it weaker." If, on the other hand, the cap has pulled a disappearing act, it probably wasn't securely attached to begin with or it got jostled loose with use.
Cure: Fortunately, many companies use the same packaging, so try taking a cap from another bottle, says Marshall. Gently press it down onto the stem, being careful to point it away from your face.
Preventive tip: Replace the cap by pressing it gently and directly on the center of the nozzle.