This isn't my usual "look what I made!" blog post... but I'm sharing it as I experienced this problem and I couldn't find anything online about it!
When I first got my Cricut Joy, I also got some Insert Cards and a Card Mat. However, when attempting to make a card, I found that the whole design wasn't cutting centrally on the mat, it was cutting too high, and therefore the corner slits were cutting off the card! One, this didn't look right; and two, it didn't function correctly - the top slits were unable to hold the insert rectangle of card. You can see two examples below...
After searching high and low on the internet for some kind of solution to my problem - what was I doing wrong?! - I called the Cricut helpdesk. They were so helpful, and after us trying a few different things while on the phone, it turned out...
...the Card Mat was faulty!!
This is something I didn't even consider! The border at the top of the Card Mat was actually too big, so when the card was positioned correctly on the mat, it was actually lower than the Cricut Joy machine expected it to be. The machine cut as it should, but because the card was lower down, it was cutting the design too high, and off the top of the card.
The Cricut helpdesk arranged for a new mat to be sent to me, and all was well with the world!
You can see the difference in the top border sizes below... the correct one is on top.
The new mat made all the difference, and I could now make the cards correctly!
I hope this helps anyone who experiences the same problem - as there must be a load of these faulty card mats out there!