What do you mean by "the score reels do not reset properly"? To help, we need a better description of what is happening:

Do they continuously spin? Stop at random or specific numbers? Do the player 1 reels stop and the player 2 reels start? Is the score motor continuously running until you switch off the machine? Is the ball ejected into the shooter lane?

I'd start at the beginning, why isn't the start relay activating when there are credits on the game and the replay button (red door switch) is pressed?

Can you test this circuit with alligator clip (or EZ-hook) jumper wires?

http://www.pinrepair.com/em/index3.htm#features
https://pinside.com/pinball/forum/topic/using-schematics-and-8-foot-jumper-wires

Unfortunately, I don't have any Big Brave schematics, but I have Duotron's (a 2 player from the same year). Locate these circuits on your schematic, it should be similar (maybe even identical), wire colors are probably different.

Untitled 2 (resized).jpg

How far does the motor rotate before it stops? One third of a turn, a full turn, multiple full turns? The start relay (S) starts the motor turning but SB2 has to activate to keep the motor turning through the reset cycle. Score reels are not going to cycle if the motor is turning off early.