That's like asking why does your Internet not work without providing much info, IPTV is not a single thing, contact the the person you are getting the service from and ask why it is not working, as you get several IPTV Providers.
Could be one of several issues. You can have the best speed but it only takes one other issue to trip you up. Things like if your base connection speed isn’t fast enough, if you’re far away from your WiFi router, if lots of other people are using your home network for data-heavy activities at the same time, or if you’re on an old broadband network that’s facing bandwidth congestion or interference. Also physical obstructions like walls can play a role. maybe the router isn't up to scratch or needs an update. Maybe your ISP is throtteling you.