I have been swamped with calls for my propane cylinder compliance booklet I have put together over the years. On a forum in Keller's on-line site, I answered questions and one of the more popular one was regarding storing these cylinders. The fact that got a lot of people's attention and request for the booklet was the position of how a cylinder can be stored. Most users store them on their side inside racks/cages. 

When we store these cylinders on their side, WE MUST BE CERTAIN that the relief valve is ALWAYS in the vapor space of the cylinder. We often cite in our 3rd party H&S audits... 

1910.110(e)(5)(iv)(b) On portable containers that may be filled in the vertical and/or horizontal position, the fixed liquid-level gage must indicate maximum permitted filling level for both vertical and horizontal filling with the container oriented to place the safety relief valve in communication with the vapor space. 

or 

1910.110(b)(10)(ix) and 1910.110(d)(4)(i)(a) states... Safety relief valves shall have direct communication with the vapor space of the container at all times. 

Take a minute or two and go out and check your cylinder storage area and see if they are all positioned with the relief valve in the vapor space (e.g. valve is at the 12 o'clock position) 

By the way, take a look at the cylinders in use and see if the worker installed them on the back of the forklift with the relief valve in the vapor space.