well trip oil pressure low is possible in the following cases
1. check to see if the liquid fuel stop solenoid ( drain control solenoid ) of the hydraulic valve 20fl-1 is properly seated. If not properly seated then oil pressure will not hold.
2. you have already checked the manual trip leaver , but make sure that it is seated as well
3. check if the hydraulic trip regulating valve VPR1 ( found in the lube oil schematic ) is passing or not. If it is passing , you will face similar problem.
4. check to see if the main lube oil pump pressure relief valve VR1 is passing or not.
these are the most common problems. yours will fit into one of this. and yeah , check it the AOP is pumping properly and the nrv in the discharge line is not drop seated.