@Mgmg 

What are the hardware and firmware version of the HS100 which cannot use schedule feature properly? 

Try to change the time zone/location to another one and change it back to the correct one to test whether it works. 

Considering you have three plugs in total and one of them works fine, you can swap their locations to give it a go.

Besides, you can change another smart phone to setup the schedule if possible.

https://www.tp-link.com/support/faq/24947/

Still the same, please tell me the mac address of the two plugs and your cloud account and model number of main router. 

Note: For the first two info, you can message us privately. 

Thanks in advance.