Restart the computer (most issues can be solved by a restart).
Make sure the browser is up to date. The DRM won't work in some older versions of browsers.
Make sure nothing else on the computer is using a lot of CPU or network bandwidth, e.g. downloading content.
Turn off the ‘Adblocker’, as this can intercept and slow down network traffic.
Try a different browser if they are on Windows or Mac (can't do this on Android or iOS).
Different browsers use different DRM, so switching browsers can often solve an issue. E.g.
Edge uses Microsoft PlayReady
Safari uses Apple FairPlay
Chrome, Firefox, and Opera use Google Widevine.
The player switches bitrates automatically based on the user's current available bandwidth. Most times this works well, but if users are having issues with buffering they can force the player to use a lower bitrate which should improve stability.
To do this, select the ‘Auto’ button in the bottom right corner of the player and choose ‘Bitrate’ (for example, the 3.0 Mbs), as shown in the screenshot below.