autopilot problem

[Guest claudia]

hello im claudia i have pmdg b 747 4000, i wish to fly from egll to flls,my flight plan has as a sid MID but when i take off autopilot does not engage. At the first attempt I got a FMC message "UNABLE NEXT ALT".Reducing the altitudes for each point of SID the above message was not repeated, but again the autopilot does not engage anyway after take off.Could it be a problem of fuel quantity, gross weight or similar ?Previously I carried out a fligth EGLL KLAX with the same aircraft and more fuel, but with a differnt SID (WOBUN) without any problem.

[FLEX1978 373]

UNABLE NEXT ALT means VNAV will not reach the altitudes in the SID with the calculated profile. Main causes are Derated CLB1/2 or high gross weight. It wont affect autopilot engagement so your problem is elsewhere.

[harvester21 34]

Are you sure you have the flight director engaged?