Efficiency with flaked corn.

My brew buddy and I are struggling with efficiency when using flaked corn. The first time we used it our volume was short, could possibly be from a longer mash that he decided to do. The second time using it I came up .010 off my OG. Has anyone else run into this issue or have any advise for me? Some other info that might help to let you know: I do all grain and batch sparge in a 10 gallon cooler and have done seven other recipes that I have hit my numbers with.

Are you sure you have right yield percentage (or ppg) for flaked corn?
Palmer says it is 33 ppg:
http://www.howtobrew.com/section2/chapter12-4-1.html

but I'm not sure it will drop your OG for 0.010 even if you put max 39 ppg (assuming you have constant efficiency)

What about other efficiency parameters, maybe there is something else you predicted..?

I just checked and the ppg was set at 38 for the flaked corn. After the adjustment it only changed a little bit. It's a step in the right direction though.