Help with Pita

I have been making pita for 10 years using the same recipe. Today, the dough was as usual, yet only three of the eight puffed up, normally. The other five puffed up around the edges of the loaves, but left a strip of un-puffed bread down the middle of the loaves. As usual, I baked the loaves at 500 F, using a pizza stone. I have never seen this phenomena. Any ideas as to why I got the incomplete puffing of the loaves? Thanks in advance,