Home button having mechanical part (switch), guess it's defective now. this guide shows how to replace your home button with touch id.
Here you can order spares ( Read the note before buy) also you can find required spare part from many eCommerce webs like eBay
if you have good skill you can repair the switch part of the home button
Until fix the fault you can use assistive touch function,
One of below ways allow you to enable assistive touch function
- Go to Settings > General > Accessibility > AssistiveTouch, then turn on AssistiveTouch.
- Tell Siri “Turn on AssistiveTouch.”
- Go to Settings > General > Accessibility > Accessibility Shortcut and turn on AssistiveTouch.
Hope this answer will help you to fix your iPhone