Was it working fine before the code update? If so, it's safe to assume that it's something in the code that changed. I'm pretty sure the new code now requires you to complete the full orbit and hit both switches in order to get credit for the shot. For example, a backhand shot to the right orbit usually doesn't go all the way around. Perhaps this is what you are experiencing?

If you were having the same problem before the update, it's likely the mylar in the right orbit that is causing the ball to "hop" over the switch. Removing the mylar will likely fix the issue.

Just following up to see if you figured out what the problem was.

Can I bump this to the top as I just picked up a GB premium and am having the same exact issue as the original post. All switches/optos test good in the test mode, but even if I complete the full loop, it’s 50/50 if it will start the mode.

Just thought I’d bump this to the top in case anyone else has this issue.
On GB code 1.17 you need to complete the right orbit to start a mode (when it’s blinking to start).
The switch on the right orbit sends the post up to hold the ball as it makes the loop from right to left. The switch on the left side close to the up post lets the machine know you made the orbit and will start the mode. My switch close to the up post was flaky at this spot so it would stop the ball with the up post but wouldn’t register that the ball had made the loop and wouldn’t start the mode.
Sorry for the long explanation and for something that seemed so easy.
Keep flipping!