Sometimes, anti-virus software can interfere with your webcam working properly. You should look at your anti-virus settings to see if there are any camera specific or "real time protection" settings you can toggle off. You should re-enable them after your exam is submitted.
Below are links to articles featuring commonly used anti-virus products, and instructions that may help you to get your webcam working with Respondus Monitor.
Note: McAfee anti-virus does not have any camera privacy settings that we are aware of that interfere with a webcam operating correctly with LockDown Browser.