Hello Willson - Welcome to NLG!
An easy way to start is to post a few photos of your machine, showing the complete front view and of the insides if there is an issue there you want to discuss. Also, there is usually an idplate located on the same side of the machine as the pull handle, it will help id the machine.
If the bill validator takes in the bill and then returns it that probably means it has old software (internally to the bill validator) and doesn't recognize the bills currently in use. Did you try a $1 bill? It should always work since it hasn't been changed, whereas the $5, $10, $20, $50, and $100 have all had security enhancements that changed the design and appearance, this means older bill validator software won't work with them.
I think many of the player pushbuttons are wired in sequence, so if one is intermittent then the others downstream won't work. If you are satisfied that the pushbutton switches and their wiring are ok then check the wiring harness from the buttons over to the main cabinet, it has a connector where it runs from the door area over to the main cabinet, check connections at those connectors. The player buttons won't work until there is at least one credit on the machine, so you have to put a coin in and have it accepted.
Locate the white TEST button inside your machine, pressing it will get you into diagnostic mode, there is a diag screen where you can test each button thoroughly.