Why does my nds load a game then tell me the game card has been removed?

Does anybody know why this is happening, I am new to downloading and have and AceKard and some games are good but others, either don't open on the nds or they say the game card has been removed and to power off and restart which doesn't help at all. Why why why, please help
Prue

It's because your flashcart is not compatible with the game. Take a look at the common issues topic.
https://www.romulation.org/forum/index.php?topic=1462.0