The lining of most wallets is leather. Leather will stretch over time. It's natural. Just like how good leather shoes loosen up after some use. I only put 1 CC in each slot and have had no problems. They're definitely not as tight as they were initially but they're not super loose either. My CCs don't fall out.
Yes, the slots definitely loosen, but they're supposed to. When I first got each of my wallets, the slots were extremely tight - so much so that it was incredibly difficult to get the cards in and out. I had to actually stick my nail underneath to prise them out the first few times! I wouldn't accept a wallet with loose slots, because that would mean it was either used or a display/item that has been shown to and tried out by other people.
Anyway, yes, it's normal for the slots to loosen but don't put more than one card in each slot or they may loosen too much!
I used to use a zippy wallet, but the CC slots ended up stretching and ripping at the edges. I guess because of repeated taking cards in and out. I use the Origami wallet now because the CC slots are reinforced and individually sewn.