I've found that pipelines doesn't kick off for the very first commit in a new repository. Doing a second commit works fine. While the workaround is pretty simple, it may be valuable to have working none-the-less.
As a new user to Pipelines, I try to get things to work with as simple a build as possible to discover how it works. A simple hello-world repository with bitbucket-pipelines.yml is enough to validate the feature. However, when I tried this out, it didn't kick off pipelines. Furthermore, it wasn't clear what I did wrong. As a new user to Pipelines, I ended up having to look up documentation, references, and be left scratching my head about what's wrong.