Does your throttle say slot=max ?
This means that the system's capacity to handle operating locos is full. The DCS100 can run up to 22 or 120 addresses at the same time. If you have the slot=max message, be sure that all locos that are not running are released from throttles or set DCS100 OpSw 44 to closed to increase the number of address slots available to 120.
The DCS100 command station that comes with all Genesis, Empire Builder and Chief series starter sets are set with a factory default of 22 slots. So, once you begin to operate significantly more locos, you may need to change OpSw 44 to closed to enable 120 slots.
How are locomotives released to clear slots for operating other locomotives?
The way locomotive slots are released during normal operation is handled by your command station's purging strategy. Command Stations run a "purge" periodically to release locomotives which have not been used for a while.
With Zephyr you are likely to run into the slot=max error, shown as "Full" by Zephyr since it only has ten slots. Check the documentation for your command station for details.
Zephyr Xtra has twenty slots so this is less likely to occur.
To release all mobile decoders from your command station set OpSw36 to closed. OpSw36 is not implemented on the Zephyr Xtra DCS51. Use OpSw39 instead. Many operators do this each time they finish a large operating session so they can start clean next time.