My SAT score is alot lower than PSAT score?

When I took the PSAT junior year(this year) I got a 2100, however I just took the SAT and got a 1970, which really dissapointed me considering I took an SAT prep course. My scores for PSAT were, 710-Math, 690-Reading, 700-Writing, for the SAT I got a 690-Math, 670-Reading, and 610-Writing(10/12 on the Essay). Does this happen often? What can I do to improve my score? Thank you