You are likely getting a lower efficiency than normal due to the Wheat malt you are using. This is the same for me. Whenever I make my hefs (7lbs wheat, 4lbs 2-row), my efficiency goes down as well.
Wheat malt is a little bit smaller than the 2-row and will likely not crush as finely as the 2-row. A couple of professional brewers I know crush the wheat malt separately (and more finely). This way they increase the efficiency without affecting the crush of the 2-row.
However, if you do this, you will likely have a different beer. I always just allow for the lower efficiency and purchase more grains to accommodate for the lower volume output.