My name is Chiranjeev. I am from Dell Social Media Team.
As the USB ports on the monitor work fine, it means the USB upstream cable is fine. It can either be an issue with the webcam or the webcam application. You can try installing the webcam application from the following link
In case this does not fix the issue, it would mean it's an issue with the integrated cam.
Let me know of the findings. Glad to be of assistance.
Social Outreach Professional
You can reach us at http://en.community.dell.com/