The key is likely "kept me" as in many people who did not requalify are kept at Plat. Being bumped up to a new level is fewer and farther between.

Have you called and asked? Did you get your birthday bonus night? If not, you could call, ask for it retroactively and see if they bump you up to Plat. Also, check that you got credit for all your nights last year. It's not unheard of to not get credit for a random night, especially of you had two different billings over consecutive nights at the same hotel. (example: Night one on points, night two billed to your cc.)