Jenkins : Why is Jenkins' SCM polling not detecting changes in the repo?

I have set up a couple of docker containers with an experimental CI pipeline (i.e. git (or actually gogs), jenkins, portainer, plus a postgres DB).

For my first project I have defined a build pipeline that is supposed to poll the repo and - if there were changes to the master branch - it should fetch that branch, build it, run unit tests and create a docker container with the built artifacts on-board.

After some substantial fiddling that whole process now finally works fine but only, if I trigger the build by hand! The automatic polling for some reason still does not work!

I have checked the "Poll SCM" checkbox and first entered "H/5 * * * *" (i.e. every 5 minutes), now - while debugging this - I even entered "H/1 * * * *" (i.e. every minutes) in the Schedule field. But for some odd reason no builds are ever triggered, regardless of how many changes I push to git/gogs! :-(

What could cause this? I mean - apparently Jenkins can talks to the repo and fetch stuff from there - so, why does the check for changes never trigger?

I am using Jenkins (v2.138.3) against Gogs (Version: There are no errors in the logs.