The normal reasons
Generally, there will be two reasons why an AUR package has "stopped working":
A repo package was updated and the AUR package was built against an older (and incompatible) version of that repo package;
The AUR package was updated and requires a repo package version not present in Manjaro stable.
The solutions to these:
Rebuild the AUR package.
- Revert the AUR package to the version which works with stable;
- Wait for stable to catch up;
- Switch up to the testing or unstable branches to get the newer libraries faster.
The other reasons
The other, less frequent reasons, are
The package file (PKGBUILD) has an issue and needs to be fixed;
The upstream software has broken in some way'
In these cases, once you have exhausted your own local testing/troubleshooting, the issue needs to be reported:
To the AUR package maintainer via the comment area on the AUR package page;
To the upstream project (e.g. on their issue tracker).
This way the issue can be fixed for everyone.
The further information (or, "But why???")
AUR "packages" are community-added and maintained package description files, not a ready-made package.
With a repo package the package maintainer maintains the package. With an AUR package you are the package maintainer, so you are responsible for maintaining the package.
AUR packages are targeted at Arch stable (so Manjaro unstable).
When software library versions change their features and way they work can also change. These changes are normally backwards-compatible, but not always, and almost never are they forward-compatible.
This means a package built with a new library version will rarely work with an older library version (not forward-compatible), and it's also the reason why "partial upgrades" can break things.
Hence, software compiled against certain software libraries needs to be re-compiled when those libraries change.
As the package file maintainer, you need to recompile the software in the AUR package file you maintain for your own system(s).