Most often the subscription is not at fault; one server or the browser cache is. Switch location in the app and refresh the subscription profile, which is enough in most cases.
Open the site in a private window. If it works there, clear the cache and cookies for that domain in the ordinary window.
The subscription carries servers of two kinds: VLESS Reality disguises itself as an ordinary HTTPS request to a real site, and Hysteria2 runs over QUIC/UDP. If nodes of one kind will not go, try the other; on mobile networks with packet loss, Hysteria2 usually holds up better.
HAPP on Windows and macOS has two connection modes. In Proxy mode only the browser and programs that can work through a proxy go through the VPN. In TUN mode all the device's traffic does. If the site will not open in a particular program, switch to TUN.
The browser extension protects browser tabs only: the other programs on the computer reach the network directly.
A second VPN running at the same time, a "smart DNS", or an antivirus that inspects HTTPS traffic often break the connection. Turn them off while you test.
Banks, government services and some Russian services refuse foreign addresses themselves. That is a separate case, covered in Russian services do not work.
Check the subscription end date on the payment page and the list of connected devices in the Devices section. The subscription link has a device counter: if the limit is used up, unlink a device and reconnect. Check once more that the app is using your current profile rather than an old import.