I would like to understand how matchmaking feels so random

I am at gold V now, I main support, sometimes I get matched with good teams on both sides, where I can really feel that my plays as blitzcrank or nautilus or whatever support can make a difference (protecting the adc, grabbing, etc), sometimes I get matched with really bad players (on both sides) that do not know how to react to my plays, and the game starts to feel so random (like 50 50 win lose, not influenced by my plays at all) that I really lose all interest in playing, since as a tanky support with no damage I have very little chance of carrying. How does it feel so random? and mind that this is from one game to another, I get that if I am a bad player I get matched with bad players, but this really not seem to be the case.