Hello @Tom29 

Thank you for reaching out to the community. 

If the smart plug can not cut off the power, you could try some basic troubleshooting tips here 

Try a factory reset on the smart device and see you can turn on/off the device by pressing the power button 

How to reset TP-Link Kasa smart switch and plug

Try a different wall outlet and home appliance on the smart device

If the above steps, does not help, it could mean some has become defect inside and usually need a replacement within the warranty instead of repairing, you could reach the retailer or our local support to check the device the warranty.