We don't automatically resolve the dependencies for Gradle when the build.gradle changes. Sometimes the resolution can be slow, as Gradle will hit the network to resolve the transitive list of dependencies. I experimented with having this run in the background, but if it was really long running, I had problems with other gradle commands blocking while it ran. So for now, it is manual.
You have a good point, it could be more discoverable. It may be possible to pop up a toast with a "click here" to update your tagging dependencies when we see that the dependency section of any of the build.gradle files has changed.