Sometimes a push does not trigger a pipeline

Summary

Sometimes a push does not trigger a pipeline to be created despite the commits being received.

(Re-opening https://gitlab.com/gitlab-com/support-forum/issues/575)

Steps to reproduce

It's intermittent.

Example Project

n/a

What is the current bug behavior?

Commits are received but after an extended period of time, still no corresponding pipeline exists.

What is the expected correct behavior?

A pipeline should be created and proceed through the stages

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

GitLab Community Edition 9.5.10 gitlab-ce@76c4568daa809c85a8de6e2df8546ef46e6470a4