Check for an adblocker
A large percentage of adblockers are installed by friends and family members with access to a shared device. Check your browser's extensions list to see if any adblockers are installed.
Privacy Extensions
Many privacy extensions also block ads in their default mode, and nearly all privacy blockers block ads when running in their more strict modes or with more block lists installed. These extensions can be disabled for individual sites much like adblockers.
Browser Privacy Modes
Browsers like Firefox and Edge offer enhanced privacy modes that work very much like adblock and privacy extensions.
School, corporate, military and other secure networks
Many institutions that provide internet for their students and employees block ads for a variety of reasons. These blocks cannot be disabled by the end user. If you believe this applies to you please contact support and mention that you may be affected by a network level ad blcoker.