While unusual, there are instances where a dimmer or switch may stop responding to pressing the on/off paddle or dim/bright bar.
In these instances, you need to power cycle the device. If it is a dimmer (DW6HD or DW1KD), simply lift the bottom of the dim/bright bar until either the light or LED at the bottom of the dimmer turns off. Wait 5 seconds and then push the dim/bright bar back in, the dimmer will restart.
To power cycle a switch (DW15S), you will need to flip the circuit breaker powering the switch off for 5 seconds, and then flip back on.
It is important that you ensure the the Decora Smart device is running the latest firmware. Once your switch or dimmer is restarted, go into the Leviton app update the firmware: Checking and Updating Firmware on Decora Smart Wi-Fi Devices
In most cases this will address your situation. If the loss of control continues or if you have any questions, please contact support at DSSupport@leviton.com.