It's probably the cart reader, make sure the cartridge has clean contacts if not do so with either a q-tip and some IPA and rub gently, if you don't have any, pencil erasers may work. Then use a flashlight (or any strong light source) to look inside the cartridge reader and see if there's any dust and do the same with a q-tip and IPA to clean it if it's dirty, also see if the pins are aligned properly in there too.
Not likely, there's a trick if doing the above fails, for R4's anyways, is opining the cart and adding some thin folded paper to it just so that when inserted it sticks tightly in there, though be careful it can get stuck, and i have no idea if it'll work in this case or not...