Allowing Incoming Connection on Bitcoin Core
I had the same problem. The port was open, but the test at bitnodes.21.co would fail. However after my Bitcoin client downloaded the entire BlockChain everything worked. I used about 144 hours to download the entire chain with a 150Mbps fiber line.

Before the entire chain was downloaded the test at https://bitnodes.21.co/#join-the-network would give an "Unreachable" error. After it downloaded it changed to Green saying it's reachable.

So you just have to be patient :)