Can’t decrypt ssl traffic

I am trying to decrypt SSL traffic between a client and my server. I have added my server's private key to wireshark, and I can decrypt data that I send from a test app on my local LAN.

I see the following ssl packets:

client hello server hello, certificate, server hello done client key exchange, change cipher spec, finished change cipher spec, finished HTTP data

But when I can't decrypt data from a different client.

I see these ssl packets

client hello server hello, change cipher spec, encrypted handshake message change cipher spec, encrypted handshake message, application data application data

There is nothing in the \data\debug_file.txt that indicates that a DH key exchange is going on?

Can someone explain why the two different clients are behaving differently.

Is there something on the IIS side I can do to force the clients to connect the same way?