Could be a glitch or E+ is completely booked. An occurred E+ seat will look the same as an occupied E- seat on the seat map. Or do you see unoccupied seats in the E+ section and they are showing as E- seats. There are a couple rows, 14 and 15 I think, on some 737's that are not E+ and I have seen some seat maps show this then it changes to E+ later when they have the tail number.