I've been wanting to post this question for a while, but did just now do some troubleshooting and got it working again on my own. (Saw your question and was going to post that I also needed to help but decided to finally go look at it myself before posting, and just got lucky that I got it to work.)
I've had dual AimTracks w/ recoil using with MAME for about 5 or 6 years now. The recoil worked great when I first got it, but I noticed about 2 years ago that the recoil only fired maybe 1/4 of the time, and half the time it did fire it was very weak. The gun itself (the "lightgun" piece) still worked fine, it was only the recoil.
This happened with both guns.
I went into the AimTrack config software and tweaked the Recoil slider, reducing it some, and the recoil began working reliably again. I noticed I had the Recoil slider all the way to the right. I ended up lowering the setting. With one gun I only needed to lower it by about 1/4 (so it was set AT about 3/4), but the other gun I needed to reduce down to a little under half.
Oddly, I can't find any mention of this Recoil slider in any of Ultimarc's documentation. I guessed it was recoil force (like an internal voltage or current limiter to affect how hard the recoil "hits"). But I don't know.
Also of note, the Test button for the Recoil in the config software didn't fire the recoil either, after I got the slider adjusted to "fix" it while pulling the trigger on the gun itself, so I wouldn't necessarily trust that Test button.
I also swapped power supplies to see if affected what setting I needed on each gun, but it did not.
What I wonder is if the solenoid is getting weaker after a few years of use, but that is also only a guess.