Sounds like that’s the problem. Most mains-powered devices act as routers, but not all. Two-wire switches don’t. Battery-powered devices can’t.
With a new installation you would normally build a solid network of routers first (typically lights and sockets), then add switches and sensors afterwards. The coordinator dongle would not usually have to connect directly with many devices, and there will be a limit to how many it can handle (though that varies from manufacturer to manufacturer).
There’s quite a lot of zigbee information in the Cookbook: