I’m excited you would love to help! There are a bunch of different ways the nixpkgs-update can fail; some of them require manual intervention, and some of them require improvements to nixpkgs-update. I’ll try to cover some of the failures that need manual intervention.
nixpkgs-update log error messages that require manual intervention
“nix build failed”
This error means nixpkgs-update successfully updated the src fixed-output derivation to the latest version, but nix-build failed. Either the package failed to build before, or the update caused it to fail. This could mean one of its dependencies is broken, a new dependency needs to be added, or something about the build process needs to be updated. The nixpkgs-update log contains the last few lines of the build output, which could be useful in seeing what went wrong, but you probably should try manually updating the package and running nix-build to see the full context.
“Could not prefetch new version URL”
Either the information from Repology.org is wrong, or upstream changed how the src is fetched. Try to find out where the new src version is obtained from and see if it builds.
“More than one fetcher in”
It refuses to update any derivation in a file that contains more than one fetcher derivation, because it isn’t smart enough to update these. These packages need to be updated manually. If you want nixpkgs-update to update these derivations in the future, try to break out the derivations into separate files, so there is only one fetcher in each file.
“nix-env -q failed to find package name with old version”
It can’t find the derivation file for the package, so it needs to be updated manually until nixpkgs-update is smart enough to find the derivation file.
There are more errors than this, but these ones should keep a bunch of people busy! If you have questions about other error messages, please ask. I’d be glad to help explain more.