Insure that there is no standard splitter in the cable line between the Preamplifier and the Power Inserter.
You cannot have any Standard Splitters or another Amplifier, in the long coax run between the Preamp 'Output/Power' port, and the power inserter "TO AMP" port. Standard splitters and Distribution Amplifiers would block the voltage from getting to the preamp. In that case, you'd get no channels.
If you do need to install/use a splitter between the Power Inserter and the Preamp, you'll need to use a DC-Power Passing splitter.
Try using another coax cable between the preamplifier and the power supply as a test. You can just run it out a window to the Antenna as a test. If that works, then permanently install the new coax cable.
If you do need and amplifier, you want to amplify the signals as little as is needed. Amplification can be detrimental to the Signal to Noise ratio. Amplification can sometimes do more harm than good.