If the vent tube has a "dip" in it between the tank and the fill spout, it will develop an "air-lock" like a trap in a drain and it will not allow the venting of the tank. You may want to try to blow through the vent to see if water is sitting in it.
Mud dobbers will build nest in the vent tube IF there is no screen over the entrance. You can wad up a piece of plastic screen wire and insert it into the vent tube if you don't already have a screen. As long as air can get by, you are good to go.
2013 Silverado 2500 HD Duramax/Allison Tuned
Aviator Wright Flyer #1919
"First In, Last Out". 11B4Y
DAV, MOPH, VFW, 5/7 Cav,1/8 Cav Assoc.
11th ABN Pathfinder Co. N7MIP