The needle is not sealing the nozzle completely when you release the trigger. Possible sources for the problem include

- There is something inside the nozzle that is blocking the needle. Remove the nozzle and clean.

- The needle has finish on it and will not slide through the packing. Clean and lubricate the needle.

- The spring behind the needle is getting weak. Tighten the fluid knob and/or replace the spring.

- The packing is bad and is grabbing the needle, keeping it from sealing the nozzle. Replace the packing.

- The needle or nozzle has worn and will no longer seal properly. Replace the needle & nozzle.

Pull the gun trigger before taking out or installing (after getting the fluid adjustment knob threaded but before you begin to really lock it down) the needle so it is not ruined by the wrong kind of contact with the fluid nozzle. And always pull the gun trigger before you take off or put on the fluid nozzle if the needle is in while you are playing with the fluid nozzle.

These packing nuts are usually made of teflon. They tighten around the needle to prevent fluid from leaking into the gun body or backing up behind the needle. This is the area on the needle where you need to lube.