If in the admin panel the number of user's devices is more than the number of devices in the UI, it means that some of them were hidden according to the tariff plan of one of the user's devices.
In the tariff plans of the admin panel, there is a parameter - maximum devices. If a user reaches the limit, he will not be able to add or track more assets in his account. In case a user has assets on different plans, the minimum value will be applied.
To see all devices again, increase the maximum devices parameter in the tariff plans assigned to the user's devices.