Same happened to me. I've found out that the white gear at the end of the right motor, that conects to the wheel and makes the blades spin, has been worn out by the harder plastic of the wheel itslef. So now they are no more in contact and thus the lower blades wont sping. I temporarily solved it pulling the white gear a bit lower but it lasted only for a couple flights more :-(

There's a lower big white gear and a an upper white big gear, if your lower blades aren't spinning take the lower white smaller gear, push it down onto the lower shaft it goes on as low as it goes. This should fix your problem.