I have again some problems at the ball trough in my Indiana ones pinball. This is a nice hobby for me just because it is very challenging... but sometimes I get too many challenges and too soon. I had some problems before but those were solved by changing the 2 LM339 and the 12 pin connector. The game worked perfectly for a couple of days then something strange appears. If I have the ball trough empty I see all opto swiches (81-86) activated on swich edge test as it should. If I take a piece if paper or anything to block the beam of these 6 optos all work perfect, I can block them one at a time or multiples at the same time and all works perfect. When I drop a ball in there it goes nicely from one opto to the next one until 86 is activated and the rest are still squares. I repeat this with the second ball and that activates 85 and so on. I put the last ball in the machine and all 6 optos are fine. Then when I eject the first ball (it does not matter if I do it by hand or if I do it with power), the last 2 optos (81 and 82) are open again like if I took 2 balls out of the machine. If the ball played is last again then both switches are activated and it looks like I have 6 balls again and the game shoots another ball in game play. If I lock the ball then there is a big problem since the game thinks I have another ball out and does not shoot a new ball. The ball is in the trough but it is not detected.
I checked all continuity on the opto trough board and all tested good, I even replaced or repositioned the U1 and U2 chips since they are socketed now but nothing solves my problem.
I used a file on the bottom of the trough but it is not in bad shape and also that solves nothing. I tried to push the last ball and it is in contact with the next ball but the opto does not see it even if it is right in his pass.
What should I check next? Probably I will go to see the diodes but I need some good advices too. Thanks

I will try to replace the balls to see what will happen. The base of the trough is in good condition, the balls are touching one another. The problem is that opto switch 82 only works if switch 81 is activated too. I disconnect the switch matrix from the CPU board and play with the wire there and the board is fine... I can trigger the switch 82 or 81 without them to activate at the same time. I also have 13 V unregulated at the test point.
I will buy a trough repair sleeve on my next order and probably a ball trough opto board and probably a 10 opto board just to have a spare one... but first I want to try to fix it... if it is something simple.

The switch under the miniplayfield was the one to blame. Thanks for the advice.