The first thing to check when you have build trigger issues is the webhook. E.g. GitLab has a separate “webhook event” for Tags which have to be enabled. The most up to date guides can be found in the README of the open source webhook server: https://github.com/bitrise-io/bitrise-webhooks
If the issue is not with the webhook setup, and you registered your app on bitrise.io a while ago, you should make sure that you have the new webhook URL registered, and not the old one. Initially we had a non open source bitrise.io/hooks endpoint for webhooks, but every new project registered should now get the new hooks.bitrise.io/ webhook URL automatically. If your webhook is still registered with a bitrise.io/hooks/ URL, just change it to the new hooks.bitrise.io/ URL. You can get this hooks.bitrise.io URL on the Code tab of your app on bitrise.io.
If you already have the hooks.bitrise.io/ webhook registered, then you should check the “Webhook history” of your service. hooks.bitrise.io/ returns a detailed error response in case it rejected the webhook, something like {"success_responses":[],"failed_responses":[{"status":"error","message":"trigger pattern did not match any defined mapping: Run triggered with params: push-branch: , pr-source-branch: prtest/t1, pr-target-branch: develop, but no matching workflow found","service":"bitrise","slug":"...","build_slug":"","build_number":0,"build_url":"","triggered_workflow":""}]}
For more information please see the http://devcenter.bitrise.io/webhooks/troubleshooting/ guide.