It is my understanding that the Andair two position valves have a "common" position that technically feeds fuel from both tanks. This is so that fuel is not interrupted from one tank before it begins to flow from the other tank (when switching tanks). If true, it is a safety feature. The difference with the OP's valve is that it may simply be labeled with a "both" position. So in this case, it may be prudent to simply order a new face plate or somehow cover/delete the word "both", and leave the valve itself as is. Best to check directly with Andair and post the findings back here for future reference.
Bevan
Ding!
Some variation on that theme likely applies to most selector valves. If the label says 'both', it *implies* it's ok to operate there, but if it just says 'left-right-off', the pilot should pick one.
A friend sold an -8 with the current 'stock' valve; 45 degrees L & R for tanks. Buyer had fuel starvation in flight with fuel in the tanks; later said that he thinks he might have aligned the handle either fore/aft or in line with the spar because he'd previously owned an RV with one of the old style valves. Nothing wrong with the valve or the label; he just didn't select a labeled position.
To the OP:
If you don't find a blocked vent, is it possible that you finally found the right conditions for the plane to draw from only one side with the selector in 'both'? Even a high wing Cessna will feed unevenly when running on 'both', though it's difficult for it to suck air (unlike low wing planes).
As a FWIW, on my -4, with less than ~1/2 tanks I can disconnect the line from the tanks to the (stock) Van's selector valve without fuel flowing.
You can check for actual flow by selecting 'off', and disconnecting the fuel line at the engine or gascolator, as others have recommended. If you use a tank leak test setup (balloon on one leg of at T fitting, or a manometer tube on the T), you can select a tank and pressurize the tank through the vent. If there are no blockages, you can easily move fuel through the valve and out the line in the engine compartment. Then select the other tank, & repeat. The balloon or manometer tube will protect the tank from over-pressure, & still allow enough pressure to move the fuel through the open valve & line to the firewall area.
Charlie
edit: You won't get 'bubbles' in the fuel by blowing in the vent, unless you have the tank completely full & that wing is 'low'. The vent is (or should be) at the highest point inside the tank.