Can't set font-weight on Google Fonts

The answer is that the Google font isn't calibrated to carry all of the weights you want with it.

Some of the browsers may display 'thicker' or 'thinner' type, but they are tricking you. They are doubling up the normal font to simulate what you are asking for when there is no actual weight of that type available. The ones that do that are trying to be nice, but these days it is more confusing than helpful. According to your code snippet, you have:

@import url('https://fonts.googleapis.com/css?family=Rubik');

vs

@import url('https://fonts.googleapis.com/css?family=Rubik:300,400,500i,900,900i');

(Google Fonts UI 2017 - may look different in the future but the concept is the same for any font service)

Many fonts only have 1 weight.