dual tank question?

[ggtsvnv]

I have a couple questions about dual tanks. My truck is an 87 SB 2wd, Silverado with a 305 TBI and a 4spd auto trans.

I have dual tanks and only my passenger side tank works. If I switch the dash switch to the drivers side nothing happens. I want to get both tanks working. My question is what is the most common thing to check for when your dual tanks don't work? Is the dash switch a common part to fail? Or should I start with the selector valve? Or some other part?

Thanks in advance