What type/model of a microphone are you trying to use? Is it a dynamic or condenser mic? If you're using a condenser mic, you'll need an external phantom power supply to power that type of mic, as it won't be powered by the Spider Jam natively.
"I have made sure that the volume is up" ---> does this mean the actual Mic trim knob by the side of the XLR input jack?
Also, in the Inputs/Level page, there's a tab for Mic/Aux/CD. In the Mic page, is the input value set to at least 0 dB?