Left arm weaker than right arm...

Hey guys,

Newbie here that has been going to the gym about twice a week for the last 3 weeks.
My main focus is cardio but I do hit the weight machines to try and get some sort of muscle on me.

I notice that when using the weight machines that you sit down on (don't know their names), after about 20 times moving them up and down, my left arm will feel way weaker than my right and I struggle to do more.
Note: I do two sets of 10 before I start to feel the weakness.

Should I focus on my left arm a bit more with single weight training or just keep at it?

Im doing around 20kg - 25kg for my arms. (forearms, biceps and shoulders).

Thanks.