Issue with build triggering

I have a weird issue, and was wondering if someone might help.

When i create a pull request, the build which is connected to my git VCS root is trigger. just like it should. Everything is fine to this point.
Now, when i update this pull request with a new commit, lets say i edit a file i didn't touch before -> build would again be trigger, which is what i expect.

Now, the issue is, when i change a file i already changed as part of the pull request, for some reason, nothing is triggered. I need to change a "new" file in order for the build to be triggered.

Anyone have any idea what might be the issue?

thanks!