Trying to install SSL: Private key does not match certificate

I am trying to reinstall SSL on a domain where the previous certificate expired. I have removed the old certificate and I am attempting to install the new certificate I purchased from NameCheap in Web Host Manager per these instructions: http://wiki.spry.com/Installing_an_SSL_Certificate_in_WHM. My problem is whenever I am at Step 9, installing the SSL certificate I WHM tells me my private key and certificate do not match. I have attempted to recreate the CSR and certificate from a new private key multiple times all with the same result.

I don't know if this is relevant but if I use the self signed certificate WHM generated instead of the certificate I purchased the private key and certificate do match. Any ideas why the private key and certificate aren't matching?

WHM attempts to find the appropriate private key to match the domain.

However, if multiple CSRs or private keys are installed for the domain, the system may not identify the correct private key.

To correct this, you can manually paste the correct private key into the boxes when installing.

You can find the various private keys on the server using the SSL Cert/Private Key manager link in WHM.