Do you see the SLA queue item created and processed by the agent? If that piece is happening, then you can trace the SLA agent to see what is happening when it processes the queue item.
If you do not see the queue item getting processed, then check the pyMinimumDateTimeForProcessing value. The SLA agent also needs to be able to lock the work object. If another user has a lock on the work object, then the queue item will be requeued to try again in 15 minutes.