Hello,
I placed a trade with an NPC located in Cerulean City: Cerulean House 5 2F. He asked for a Clefairy in exchange for Cleffa. I submitted the trade and I did not receive the Cleffa. I made sure to double check my starting line up and my Poke storage several times. I also waited an extra day and logged a few times and the Clefairy is still missing and I was not given the Cleffa.
I found the error:
The Cleffa was in my PC but it will not allow you to search for it or alphabetize it. It was on a page by itself in my box so if you are having the same issue be sure to check other pages!