Sorry for the late reply, I thought I would get an email notification for this.There is not enough voltage at the gates of M2 and M3. As a consequence, after the voltage drop M4 and M5 saturate and can't pull enough current to forward bias M8 and M9 and they saturate as well.
by Duane Benson
by Jake Hertz
by Duane Benson
by Duane Benson