Acetylene & Oxygen Tank blues

[Weasel Diesel]

Guys, I'm having a hell of a time. My gas torch tanks keep leaking out somehow. I make it a positive point to turn off the main tank valves when finished. Now, the tanks are on an exchange program, so the tanks get swapped out for different ones when I go for a refill. I can understand if I was dealing with the same tank everytime, but I'm not. This is a portable torch set, so it has the smaller tanks (20cf oxy & 10cf acy ). New tanks were last installed in july, torch was used 1 time to cut off exhaust system, then put away. This morning I went to use it and yep, empty. So do you guys think there is a problem, or do you think I am expecting too much out of the small tank system? Soapy water test done about 4 times and no visible bubbles. I have been getting tanks from Tractor Supply store.