I think your start up point something is wrong:1. If Vbg point initial is high, could not startup
2. If startup device both in satuation, how to startup
If you are using Cadence IC614 there could be a bug. Some times the simulation shows that current does not flow through the paths with ideal current sources. This could be observed with current mirrors. so once replace your current source with a resistor, generate netlist and redo it.