I asked this question a while back, and got a lot of good suggestions. But they didn't fix the problem.
My most recent batch is a witbier. It started at 1.050 and started fermenting in 12 hours. The airlock bubbled rapidly for five days and then stopped, even though I raised the temperature to 72F after the third day. After a week I took a gravity sample, and it's still at 1.028.
Based on the advice in the last question, I used a 1.2L starter this time and aerated my wort with an aquarium pump for 20min before I pitched.
Witbier recipe (5gal batch): 7lb Pilsner, 5lb flaked wheat. Mashed for 60min at 153F, temp dropped to 152F by the end. Wyeast 3944. RO water with 3g of CaCl2 and 3g of CaSO4.
I never had this problem in my first 10 batches. Why might it be happening now?