Delayed turn on / off on scheduled time

I got 2 Yeelight LED colour bulb in my bedroom and I scheduled them to turn on at the same time, as well as off at the same time. It works perfectly fine for the first few days, but recently one of the bulb started to delay more than 30 secs to turn on and off.

May I know why this happening ? Any solution ? Both are connected to Singapore server, with latest firmware.