There can be multiple reasons why your workflow is unable to trigger. Some are related to HubSpot restrictions and some to the working of CloudFiles.
File not in Contact context
HubSpot only allows external apps to trigger workflows in the context of contacts. Make sure the file you are viewing and trying to trigger to workflow on, is uploaded in a contact record in HubSpot.
View the file in Incognito or in a different browser
The CloudFiles viewer is smart. If you open the file without incognito mode(using "preview" button), the viewer will detect that the uploader of the file (you) is trying to access and won't record the session.
Workflows trigger after 1-5 minutes
Just so HubSpot APIs are not overwhelmed, CloudFiles triggers workflows after a certain amount of time of file getting viewed. This may vary from 1 to 5 minutes. As soon as the event is triggered, a timeline post will be created in the contact. Your workflow should trigger within 5-10 seconds of the timeline post. Check out our Advanced Quick Start Guide to understand how you can enable & use the CloudFiles timeline post as a sanity check for your workflow.
Sometimes you may upload a file and use it for a few days, thus gathering a certain number of views on those files. If you decide to create a workflow on a certain condition, make sure that condition is not already met by some contacts otherwise HubSpot will not trigger workflows for those contacts. If you still want to trigger workflows on those contacts, you must enroll them while creating the workflow. This way, as soon as the workflow is created, it will be automatically triggered for those contacts and your actions (e.g. creating a task) will occur.
If you still face issues triggering your workflows, please create a ticket or send us a mail detailing the workflow you are creating. We will get back to you as soon as possible