I have a Williams AZTEC and it is also not awarding games for High Scores. AZTEC is a 4 player so a little more involved to a point. I could not find a schematic for Hi-Lo ACE online so I do not have that reference. High Scores on AZTEC are awarded through the 100,000 and 10,000 Score Reel circuit board spider arm contacts. The Schematic picture above shows that High Score is award on the 10,000 score reel.
Today I dug into it since I saw your post and thought I would get my AZTEC High Score problem fixed. As Wizard mentioned above, check your m&b (Make or Break) contacts on the game over relay. Hopefully you have the schematic to the game.
I think I have traced my problem to the player 1, either 100,000 score reel contacts or the 10,000 score reel contacts. I believe this because Player 2,3 & 4 are awarding High Score when I put the game on 110,000 score on player 2,3 or 4, shorted out the 10,000 point relay contact to simulate a 10,000 point match in the high score circuit then took the lead that awards game credit at 110,000 and touched the jones connector to the correct plug in the head unit. This completed the circuit and awarded a high score game.
This is where I stopped so I could post this. Next I need to remove the player 1 score reel and clean all the contacts and board that are associated with the High Score circuit. I believe this will solve my problem.
I will let you know if this fixes it.