I've seen a few posts about Cory's dying and I include me on the list of unexplainable Cory deaths.
Being new to the aquarium scene, I had four out of six die which I attributed to my inexperience. Then I bought two more (wanted to get the two other Cory's classmates), one died. I didn't bother taking the fish back but I was beginning to suspect the Cory's were not as easy to take care of as I was reading.
Not wanting to give up and before I bought more, I tested my water, all was in order. So, I bought three more. They went into the quarantine tank - nice and cycled too. All seem well, lively fish, no apparent illness, etc. Then a few days later - two went to fish heaven.
Went to back to the LFS with dead fish and my water. It tested well, explained my tank setup, they were at a lost and offered a replacement which I declined and received store credit instead.
Now ALL the other fish I have bought have survived and others have commented that my fish look good and active. The two original Cory's are doing find - it's getting them classmates that is providing difficult.
My next adventure is to a different LFS and before I buy, ask if the two stores have the same supplier.