My 21" mower will not start. It is not self-propelled. The blade will start to spin when I go to start it, but will quickly stop and the flashing yellow light will come on. This has been happening for the last few times that I've mowed, but has gotten progressively worse. After some tries, it would eventually start. The last time it took over ten minutes. I tried longer than that today, but it kept stalling out immediately.