Aspire timeline ultra m3 isses

Hello, In my Laptop, the Sleep button (Fn+F4) is not working and I always had to use windows start button >Sleep. I used a Acer TimelineX earlier, in which pressing the power button goes the system sleep. But in TimelineU M3, when pressing the power button, it shuts down the system.
I use the sleep button very frequently, instead of complete shutdown. Any idea of this issue and solution?