In the new 2018.1 release of TeamCity they have altered when and how .nupkg files are index for their internal nuget server (Using TeamCity as NuGet Server).
The Octopack-Teamcity plugin use to handle this seamlessly.
However after the 2018.1 TC update only one package per build seems to be getting added to the internal feed (we often have have numerous packages created). The artifacts are still correctly being generated and can be viewed in TC, just not being correctly indexed for the internal TC nuget feed and are not visible in/to Octopus. This is after ensuring the nuget feed is enabled and the re-index packages build feature is on for the projects. The work around has been to add Nuget publish package steps to the build configurations. Hopefully this report is helpful (and its not just something specific to our environment), please let me know if I can provide further information. Thank you.
Versions: Octopus 4.1.5, Octopack 4.34.0, TeamCity 2018.1 (build 58245)