My 3ds doesn't read all 3ds cards, but only ds ones.

Hi, I bought a 3ds second hand, and it was working fine, reading all cards, but recently it almost exclusively reads normal ds cards, but not all of my 3ds cards. Sometimes it says there is no game in the game slot, even when I've inserted the card, and sometimes it shows that the game card is inserted, but it says that the game cannot be read.

I took it to a local gaming store, and the person working there says it might be the console's memory, but even he wasn't to sure with that diagnosis.

Only thing that I can think back to that might have some roll to play in this, is when my cat once knocked it off late one night while it was charging, but it worked fine after that for about a month before the problem started.

Any advice? I'm not sure that to do.

(EDIT) : So, after going to a game store, the employees found out what was wrong. It was the console's region. I am pretty new to owning a 3Ds, and didn't know about this. I'm just glad it's not faulty or broken. Thank you for the answers I got!!! I really appreciate the replies!

TL:DR ; If your 3Ds is reading all Ds cards, but not 3Ds ones, maybe check the region.