Is it common for people to raise their ACT score after retaking it?

You should increase just from becoming comfortable with the test. I took it in february with barely any prep and got a 27 like you. However, this time I got a 31 with studying. I suggest getting The Real ACT Prep Guide and reading over strategies and doing all 3 practice tests. Even now I'm not satisfied with a 31, so I'll be busting my you know what to get that 32/33 in June.

Generally when you retake a test you have a better idea of the things on the test and how the questions are asked. ACT and SAT tests are generally the same from exam to exam and if you have the ability to remember the questions that stumped you the first time around and have taken the time to figure them out, you DO do better. But, and this is the piviotal point there are generally two versions of the test given at the same time. You may not get the same test as you had the first time around. So, to prepre for a retest, go down to the book store and get two different ACT practice books. Work through both. Plan to take the test a few months along from the time you took the first one so that you can relax into the knowledge of the tests and realize the question areas you had trouble with are not monsters but defeatable. That's why you get two kinds of the test to see and get familiar with. But, you can do better if you prepare for the second one and don't go in cold.

just study and you will be okay.