Re: DC Amp Biasing
First of all, did you mean your opamp design is connected to Vdd=12V and Gnd? The main reason why your design is failed is due to some transistor is in the off region. Eventhough you can get some result when you decrease the input near to 0V, maybe some of transistors in the linear-region! We don't want transistor in linear!!
If you need to bias the input, make sure your opamp itself include the bias when you design your opamp! If you just add bias in the input, your opamp maybe will not function as expected!
My suggestion is make sure all the transistors are in saturation in your simulation at every input voltage.