If the air line will not stay in the fitting, the air line or the fitting may be damaged. Sometimes re-cutting the air line and reinstalling will help with sealing and keeping the air line in. If this does not work the fitting should be replaced. This kit uses a 1/8" NPT Thread in the air bag and 1/4" air line tubing. If the fitting does not stay threaded into the air bag the bag and the fitting should be replaced.
FIR3128 would be a suitable replacement.