unable to decrypt SSL traffic using private key

"Passive" decryption with access to the private key can only work with the RSA key exchange. In other words, if the client and server negotiate with anything other than RSA (i.e. anything using Diffie-Hellman) then it cannot work.

If that's the cipher string in the SERVERHELLO message, then you are indeed doing an RSA key exchange. So then I guess the next question is how are you doing the SSLDUMP? You have to start capturing at the beginning of a new SSL session, as a resumption or renegotiation will be encrypted with keys that you won't have. Try this:

ssldump -AdNn -i [VLAN or interface] -k [path to private key] port 443 [and any additional filters]    

Does the private key require a passphrase? Do you get an error when you try to capture, or just not see any plaintext?

You can force the F5 to use specific ciphers in the Ciphers block of the client SSL profile. Try this:

AES128-SHA256

Okay, so a few things:

1. I'm assuming you did, but asking to make sure, did you substitute the bracketed values in that command with real values? Here's another example:

   ssldump -AdNn -i 0.0 -k mykey.key port 443        
   

2. As I mentioned earlier, you have to get in front of a new SSL session. Renegotiations and resumptions will be encrypted with a symmetric key that you won't have.

3. The private key you're using here has to be the same one you're using in the client SSL profile, which I'm also assuming is RSA-based.

Okay, try one more thing. Copy that private key to a local file and see what happens.