Why i had to use $forceUpdate() in many things to work in Vue?

I dont have a example in special, but many data variables or templates simply doesnt update when i change something via a button click or via some interaction with the web page, so when this happens i use $forceUpdate() method in my code and it simply works, why this happens? Why i had to use $forceUpdate()? Vue cant just detect that something changed by itself?