Why is my rotary cutter not working for me ?
 

More of a what am I doing wrong question ?
bought a top of the line rotary cutter some time ago, used few times until the first second and third time it veered away from the ruler and ruined a strip of fabric.
Put away for a while, watched tutorials and people using it ever so effortlessly, took it out again...
nope, still the same problem.

I was taught to point the rotary down to the floor and cut.

If you're cutting away from yourself, you probably have a tendency to lean out away from the ruler.
Try "walking" your hand up the ruler, and you do it like this: Put your hand on the ruler (I usually place my ring or pinkie finger half on the edge of the ruler and half on the fabric). Push the rotary cutter up until it's even with your hand, and stop. DO NOT MOVE THE CUTTER. Hold it steady right there. Now on the hand that is holding the ruler - lift up the thumb only, not the other 4 fingers, and move it up right under your 4 fingers, and set it down. Once it is set in place, then move the other 4 fingers up the ruler. Think of it as playing itsy bitsy spider up your ruler. Once your ruler hand is set, then continue cutting with your rotary cutter until it is parallel with your ruler hand again.
It sounds complicated, but once you do it a few times, it gets easier and faster.

Thank you @Peckish definitely going to try that.

Also make sure that you are not trying to cut an arm's length in front of you. Stretching to far out lessens the control you have on the cutter and can allow it to veer away from the ruler.

Donna Jordan uses a 5 pound hand weight on the farthest part of the ruler to help her hand control things. It's usually not the cutter but the user. Ask me how many strips I have ruined!

Be sure that you are sitting/standing directly in front of the ruler. Be sure you are UP and ON the ruler.

What size cutter are you using? I feel like I have more control with a 60 mm cutter.

A favorite helping hand tutorial for cutting. Check the position of her hand at about the 12 minute mark.

https://www.youtube.com/watch?v=0lgmT5BEc4M

Another one for you that shows hand positions.

https://www.youtube.com/watch?v=oAZrOFkvbRE

I took a class a long time ago from Omnigrid rep. She had us cut with different size rotary cutters. I found I liked the 18 mm size the best. I had more control and no problem at all going through four layers of fabric. And the blade are much less expensive. I liked the Clover brand the best for the small cutter. I do use the 45 mm but when I go back to the 18 I wonder why I have the 45.