Designing simple class a amplifier using mosfet
Why is the signal not amplified? I realise that the answer might be
  complex


Maybe not too complex...

Class A amplifiers have to have a standing bias current that is of the same order as the load current. See the note I've added on your picture: -



So the 560 ohm resistor forms a potential divider with your load (8.2 ohms) and the biggest peak voltage that can be produced is: -

$$\text{12 volts}\cdot\dfrac{8.2 \text{ ohms}}{8.2 \text{ ohms} + 560 \text{ ohms}} = \text{173 mV}$$

I'll leave that to sink in.