then u have two cases,
case1: for a pmos to turn on, Vds≤(Vgs-Vt)
Vd≤(Vg-Vt)
Vd< 12 - 0.4
Vd< 11.6
so for a 0.18µ tech...Vd< 11.6 volts ,
other wise the probable cause is the noise margin, u have to make the sizing such that it is minimum with respect to NM.
If u see a NM curve it mite tend towads the on stte even when your Vg is high, so work out with the sizing of the particular transistor and also otherparasitic connections attached to it...lemme know if that works...
if not we can check some other ways