If your left click is taking command properly it would be due to carbon inside left click button.
After using long time it creates problem.Simply open mouse like as shown at up side. Take a small drop of petrol or thinner into button and continuously click by finger and repeat it twice or thrice. Now leave your mouse for dry liquid. Screw your mouse and use it. It will work fine.
I have fixed many mouse like this in my home and friends.