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.
"There is no strong beer, only weak men"
"Pretty women make us BUY beer, ugly women make us DRINK beer" - Al Bundy
"Give a man a beer, he'll drink for a day. Teach a man to brew, he'll be drunk for the rest of his life."
Primary: Hoppy Wheat, Porter
Kegged: Reaper's English Mild (OO), BBK Jr., Lagunitas IPA, Fireman's #4
Notable Empties: Oaked Black IIPA, BBK I, Red IIPA, Burning Bush, Apophis "The Destroyer", Vanilla Porter
On-Deck: The Titan BW, Ale of Olympus