Reasons a NGPC cart would fail?

I recently bought Sonic and Metal Slug for the NGPC from the same seller and I can't get either of them to work. I have two consoles and all my other games work on both systems, these two games won't work on either.

Is there a common reason that these games would fail? It seems strange that they're both from the same seller and neither work.

Clean the cart edges by using contact cleaner and a qtip.

Thanks for the advice, Xian Xi. Apologies, I probably should have tried this before posting. I just thought it odd that both from this seller don't work but all my other games do.

I have some IPA cleaner aerosol here at work that I will try this evening.

Try to adjust the cart insert level