It's been 18 days and you dropped from 1.052 to 1.028? I'm a bit surprised since you made a starter. What was the use by date on the vial? Did you notice signs of life when you made your starter (foaming, cloudiness, etc.)? When you made your starter, did you leave it at room temp the entire time, or did you ferment for some time at room temp and then put in the refrigerator?
Your low attenuation may be due to the temperature fluctuations. Usually if the yeast experience a drop in temperature, they'll drop out and stop fermenting.