WireShark relies on a few heuristics to decide how to decode a stream. One of them is the port number.
The first item to check: is the RTMP using the standard port 1935 in this capture? If not, tell WireShark how to decode it (see last step below).
The second item to check: turn on heuristics Edit → Preferences → Protocols → TCP → Try heuristic sub-dissectors first.
If all else fails, brute force: right-click one of the packets → Decode As... → RTMP.
