Azure PR build won't queue in pipeline

We are running a PR build on our repo, prior to merge. Its triggered by any merge to the main branch. This is configured in the Repo Policies tab. The repo is structured so the solution and project file are in the same folder.

Now we need to add a new project. So to start, I created a folder for the existing project and moved everything into it except for the .sln, .vs, .git files and the .gitignore and .gitattributes folders. I can build and run the app and it works great.

I pushed the changed to Azure and created a PR. The PR build is set to automatically run but it won't queue.

I've clicked the "Queue" link and it doesn't do anything and then reappears after a while. I've pushed another change to the same repo as a test and it queues fine. I assume it has something to do with reorging the project? I didn't update the YAML file with the new path to the project file so I expected the pipeline to fail, I just pushed this change first and noticed the pipeline didn't run.

I've done searches and not found anything like this but I'm probably doing the wrong search. Any suggestions are appreciated.

I can reproduce the issue when my source branch doesn't have the yaml file of the build validation pipeline. Clicking on the Queue has no effect.

You can add the yaml file of your build validation pipeline into your source branch. Ensure the name and path of the yaml file are the same as those configured in your pipeline. When the pipeline is triggered by PR, the actual yaml used is the one in your source branch.