Let's get the obvious thing out the way... I'm sure you didn't, cos you'll have realised from the other carts... but make sure you're putting the game in with the sticker facing inward (ie, not visible when in the lynx)?
I only say this, as those flat carts are the only ones where you can do this, and I sold some on eBay once, and the person complained that they didn't work. When he sent me a video to prove it, I could see that he had put them in the wrong way round!