I had the same problem when I first got my 5 gallon one. You gotta make sure that black top is on TIGHT. If any air can get through the lid all the water will run out. Sometimes I think I have it tight but come out and all the water ran out. I make it so tight that I have a hard time getting it off when it is time to refill the jug.
I am always searching for a good water solution. I am going to be trying those little water cups next for my new coops. I like the idea of water not standing around growing algae. Might make it hard when I want to give meds or wormer but suppose I will figure it out.
I have 2 of these and one had the same problem when i got it. the o ring was too big ...i took it back and they gave me another o ring all it needed no more leak..of course the outlet nozzle has to be on the low side if the pan is not level. hope this helps good luck