If you have recently replaced o-rings on the tip of the hose, be sure that it is placed only on the first indentation closest to the hose end. The second indentation is for the spray gun clip to latch on to.
If there is an o-ring on the second indentation, remove it and try again.
Be certain any o-rings used to replace missing or damaged pieces are from AR Blue Clean and the correct kit for your machine as they are designed for our machines specifically.
One more possibility: In most cases, this problem is restricted to the AR240, AR260, AR383, and AR388 models.
The problem arises when the vinyl sleeve gradually moves up toward the brass tip until the silver fitting below the brass tip is no longer visible. The vinyl sleeve then prevents the brass tip to fully inserting into the spray gun connection.
The fix: twist/pull the vinyl sleeve down until you can see the silver as in the picture: