iPad Air Not Charging on Left USB of Retina MBP '15
Go to the Apple on your Menu Bar and select "About this Mac".  You will get the System Information app.  Select System Report.  On the left side, under Hardware, select USB, then find all the available outlets.  If they are all similar, you may need some hardware diagnosis.  If one is significantly less than the others, that is your answer--it just can't pump out enough juice.