Tgormley: That is right. The sap in a 3/16 line will not move unless the trees upstream are letting sap or air/gas into the system either from the tree or a leak. In a bigger diameter drop, you can see the flow since it is depart from the air in the tube and either drips down or runs down the side of th tube.

If the trees aren't running, the sap won't move. The exception is when you have a plugged up line. Your quick check by opening the line briefly will let you know if that's the case. If the sap runs down with an open top fitting, you aren't plugged.