I think evaluating your crush would be a good simple start. No use going though a long process of trial and error on other things, when just taking a look at the crush could be done in about 10 seconds. So, I think it's the best place to start.
I had the same issue you did. I did some searching on here and determined the likely culprit was my crush, as I had a good number of fully intact grains.
Here is a good resource:
take a look at that and see how your crush compares. Unless your temps are way off, I have a hard time believing that your efficiency would be that far off.
I'm not saying the other ideas are bad ones, not at all, I just think that without a good crush, none of the ideas anyone has given you here, will matter at all. If the crush is good, then take the other steps.