It's very common to get uneven amounts from your breasts when you pump. This is sometimes called a "slacker boob."
While some people don't like the appearance of lopsidedness or getting less milk from one side - this is normal (more than 90% said they experienced this on the most recent poll I did). You don't need to do anything to fix it unless you want to.
If you decide you do want to try to even things out, here are some things that you can try.
1) Pumping a few extra minutes on your slacker side at the end of your regularly scheduled pumping sessions.
3) Power pump just the slacker side. (Power pumping mimics cluster feeding, and to do it you pump 20 minutes on, 10 minutes off, 10 minutes on, 10 minutes off, 10 minutes on.