One thing is what was the temp of the wort when you took the reading? A 1.050 reading with wort at 100ºF has an actual gravity of 1.056.
Also did you sparge with a total of 3.2 Gallons? how long did you let it flow through? The slower the better, I typically sparge once for 45min to 1hour.
I did a little calculation too and with a grain to water ratio of 1.25 (12lb/15quarts) that you used, you should have used 3.75 Gallons @ 170ºF to mash and 4.8 gallons of water to sparge , for a 5 gallon batch.
Hope that helps, I'm new to this too. I have about 20 AG batches under my belt and I'm finally getting above 80%