I used to have a couple of jumper cables with alligator clips. I don't know where they are anymore. I gotta get a couple of them.Back when I just started working on stuff I had the same problem. I didn’t know how to hunt it down, so I simply added an extra ground wire to everything that needed to be grounded. Lol.
you can also solve this with a jumper wire with alligator clips. Again, something was grounded and isn’t anymore. Just find where it’s not and you’re gold.