There are two options for when snap packages get built:
A build will be scheduled automatically once a change to the top-level source branch for the snap package is detected.
If there has been a build of the snap package within the previous hour from the source archive, the build will not be scheduled until an hour since the last build from the source archive.
If the snap package has been built within the last hour from a different archive using the "Request builds" action, this will not delay the automatic build.
If you really want the build to happen before the one-hour period is up, you can use the "Request builds" action.
Builds of the snap package have to be manually requested using the "Request builds" action.