The most likely source of this problem is that the dimmer switch that can’t be added thinks it is part of a z-wave network already. The manufacturer’s instructions include the steps to reset the device to factory settings. You have to do that, and what it is depends on the manufacturer. If there aren’t such steps, let us know what model of switch it is.
In the IDE check My Devices, and make sure that all of the devices shown there exist in your network. Sometimes excluded z-wave devices hang around, and need to be deleted.
Do you happen to own an Aeon Minimote? It can be quite handy for removing and adding z-wave devices.