If your system is leaking at the connection between the tubing and the fitting, make sure your fittings are properly snug. If you have slip-lock connectors, see below.
Once you’re checked your fittings, make sure your water pressure is appropriate for your system. Out basic 3/8″ mist kits need between 45 and 80 PSI. If you are using slip-lock fittings, they will work better at higher pressures by design.