You'll need to find the relay and/or switch that is sending power to the score motor... Do you have a schematic for the machine?... Check to make sure none of the coin chute switches are stuck closed. Also, make sure the switch at the outhole is clean and adjusted properly...

I'll need to see a high quality scan of the schematic from (for example) Staples ($2) or Kinkos ($6)

Hi-Lo Ace features the rotary trip bank (seen along the left side of the bottom board). It is also a part of the reset process. When you start a game, does it rotate at all? On the schematic, they refer to it as the "reset motor" and relays with "Trip" in their name.

Just because your score reels go to zero does not mean the game can "see" that they are on zero. Tighten screws for the switch stacks on the score reel drum run out switches and then verify gap and clean and adjust if necessary.

Due to the rotary trip bank, as HowardR requested, it will be difficult to suggest places to look with seeing a schematic. The game system will not be like "ordinary Bally Single players" of the time. It may match Bon Voyage or Odds and Evens fairly closely though.
hlacebb (resized).jpg

This, from Odds and Evens. Does your Reset relay stay energized? If so, its probably a score reel not opening its zero run out switch.

oddsevenrst1 (resized).JPG

oddsevensrst2 (resized).JPG