Far as I can tell you're not doing anything wrong! Just curious what is your current rank?
If it helps, I'm currently Oberleutnant zur See (Lieutenant Sr) and my career stats as follows after my 32nd patrol:
Patrols - 32
Renown - 15,771
Start Date - 9/1/1939
End Date - 10/11/1942
Tonnage - 776,986
Medals - All of 'em, case is full!
Yet I still haven't reached Kapitänleutnant rank.
I don't know, but I imagine promotions are partly based on the realism setting. I'm playing on about 55% (my math skills just aren't up to doing manual plots
)
Here's a link giving a little info on the time needed to reach Kapitänleutnant in the real Kriegsmarine:
http://www.uboat.net/men/ranks/kptlt.htm
HTH!