I'm guessing there are a bunch of people who never switched from the old to new wallets and now their coins are lost. From what I understand, you must install python, bip32untils, and use a script to get your private keys from an old wallet. Then import them into a new wallet. It doesn't seem very easy.
I also posted in the counterparty forum, unfortunately clearing my cache didn't do the trick. This response seems more related to missing addresses not old wallets.
It seems this might be the solution if you have the patience. I know Cryptonaut does a lot around here but I'm not sure people will be able to find the support they need to recover their coins. More info about the process here
https://forums.counterparty.co/index.php/topic,507.0.html