These sites rely heavily on pop-up ads. Sometimes, aggressive ad-blocking software can prevent the video player from loading entirely.
Many internet service providers (ISPs) block unauthorized streaming domains. If the site won't load on Wi-Fi but works on mobile data, your router settings or ISP may be blocking it.
Over time, cached data can cause site elements to fail. Go to your browser settings and clear your "Cookies and Cache" to refresh the site connection.