[Airin Kawasaki]

As none of the suggestions posted here solved my problem, I made the same question to my Ai teacher today, as I was in the same situation.
My teacher’s answer was simple and correct:

Go to “Document Raster Effects Settings” in the Effect Menu.
The resolution might be in 72, so change it to 300 and that’s it!

Hope this helps you, though many months have passed since that.