Try these steps I compiled maybe they can help you.
How To Troubleshoot Your Flashcart:
STEP 1: Make sure you're not using the card reader that comes with the flashcart. Buy your own. Always use the "Add/Remove Hardware Safely" function before removing the reader.
STEP 2: Check your microSD card for smudges on the contacts. Clean with a QTIP. You can add a bit of alcohol onto the QTIP, if wanted.
STEP 3: Check your Flashcart for any abrasions or smudges on the contacts. Clean with a QTIP. You can add a bit of alcohol onto the QTIP, if wanted.
STEP 4: Possibly, your Flashcart is not making perfect contact the DS. Try the "Paper Shim Trick" to close the gap and make the space tighter.
STEP 5: Copy all of your games and saves off your microsd card to your computer (BACKUP). Download the latest "Panasonic SD Formatter" and do a full format with this application. Install the latest firmware again from a different/reliable site. Then add on your games and saves to the newly formatted card.
STEP 6: If the latest firmware doesn't work sometimes downgraded firmwares can achieve working results. Try downgrading, if it doesn't resolve just go back up to the latest firmware again.
STEP 7: There are times where the official firmware for your specific cart isn't up to par with the alternatives created. Try a legit alternative firmware for your flashcart. eg, (DSTT/R4 -->YSMENU), (Acekard--->AKAIO), (M3 DS Real---> Sakura) so forth..
STEP 8: If you're using cheats maybe they are faulty. Get rid of them and get them from a reliable source.
Or just dont put them on period.
STEP 9: Possibly the "GAMES/ROMS" you have are faulty. Obtain ones from a new location...
STEP 10: Try another DS. If possible.
STEP 11: If all else fails, get a new Flashcart and/or MicroSD Card from a reputable site.
Tell us if it works or not~ Good luck.
