Thank you for contacting the Instructure Community. We're sorry to hear you've been experiencing issues with student annotations. Presuming you've set up the file type as "student annotation" and not regular file submission or text entry - they should be able to use them. Is this affecting all students or just some students? Have they tried basic trouble shooting like clearing cache, trying another browser or device, etc.
Here is the Canvas guide:
If after all this they're still experiencing issues, it would be best if you contacted Canvas Support directly to pinpoint the source of the issue and provide a resolution: