F150 Bi-Fuel Starting Issue

[mohammond]

My 2001 F150 Bi-Fuel has recently begun having irregular starting behavior. When manually switched to Unleaded, the starter will engage and spin the engine, but it appears no fuel is entering the engine. Then if I select CNG it will promptly start. Then if I re-select Unleaded, it will generally start fine and run on Unleaded, though today it won't start at all, regardless of fuel setting. (CNG is empty now)

Ideas?