Gitlab CI/CD does not trigger automatically when merge request is created OR code changes are detected

Question

We are currently using Gitlab CICD pipeline but it is not automatically triggered whenever the developer push some code changes OR when they create merge request. Is there any method to automatically trigger the pipeline once any merge request is created by the developers?

Waiting for your kind response.

Thanks

Currently we manually merge by clicking on merge and then pipeline is triggred. We want to automate this.