Page 11 of the "GWX 2.0 The grey Woves Expansion" manual under the section entitled "Known Silent Hunter III issues and workarounds", bullet point number one:
"If you upgrade your U-boat before the first patrol of a new career, you may not receive any credit for ships sunk during the first patrol, so you should not upgrade or change anything on your U-boat on your first patrol of a new career before leaving port."
If this is not your first patrol, notice bullet point number 2:
"If you upgrade your U-boat at the end of the current patriol you may not receive any credit for ships sunk during the patrol, so you should only upgrade your U-boat when you are ashore and about to leave for the next patrol."
If you made any changes to your U-boat before the start of the first patrol in a new career... such as adding a different sonar, giving yourself an emblem on the conning tower etc. then you may not receive any renown for that particular patrol.
RTFM
