I was actually about to post this asking for help, but I decided to try one more thing and it worked, but I thought I'd share it anyways.
So I just got a new Wisdom Tree game in the mail - the ever-rare NIV Bible & the 20 Lost Levels of Joshua. The seller on eBay said the game didn't work, so he was selling it as-is. I figured I'd give it a shot. I tested it and sure enough, it didn't work. On my original Game Boy it powered on but nothing came on the screen at all. On my Game Boy Color it didn't even power on, not even the power light.
(Wisdom Tree GB games are these btw for those that have no idea what I'm rambling about)
So I tried cleaning the contacts, no change. Then I opened it up to see if there was any obvious damage to the board, and everything was fine.
On a hunch, I decided to insert the game without the front of the cartridge case (just the board and the back) and it worked perfectly! In fact, I tried it multiple times on both Game Boys and it worked every time.
At first, I thought the problem was being caused by the front half of the cartridge case, so I thought I'd swap the front half with the front of my Joshua GB cartridge case (which is a game that works just fine). Surprisingly though, it didn't work. So I thought problem didn't seem to be with the front half of the specific NIV cart, but rather with there being a front half at all.
Worst case, I didn't mind leaving it as is, since I mostly got it for my GB collection, but there was a nagging part of me that wanted to understand what the problem was.
Just before I posted this originally to ask for insight, I thought I'd try one last experiment and swap out the back half of the cart cases from both games. Sure enough, it now works. In fact, both of them work!
I guess the issue had to do with some kind of pressure caused by that back half when attached...but only with the NIV's board?? I'm not very technically minded and don't know a lot about how these systems work, so I'm just guessing.
Anyways, end result is I snagged the rarest Wisdom Tree GB game without breaking the bank!
