Takeoff

Check that the auto pilot is not switched on and check your trim wheel is set to about halfway(just below halfway trail and error)
Hope it helps

Check your control sensitivities for one thing, and get an outside view of the plane so you can see what position the elevator is in. Some airplanes take a lot of up elevator input to be ready to rotate.

You may find that very large (heavy) (Antonov AN-225) aircraft or poorly made AI craft cannot lift off because of either their construction, weight or most likely your video card, it may not be "good" enough or the drivers are old or corrupt.
Why don't you list your system specs especially your video card and what drivers your using.

http://library.avsim.net/esearch.php?CatID=Root&DLID=35457

and be sure that your flaps are set to the appropiot position. (DEPENDS ON THE AIRCRAFT)

Make sure your trim is in the right position and is not all the way down.

Don't fold the wings and try to take off with the wings folded. The aircraft just won't take off.

Note: Folded wings is not winglets.

Make sure the trim is set for T/O, AP is off,power is set cerrectly and flaps are set and make sure you are not over the max gross waight and the spoilers are retracted.

Ooh yea in the before T/O check you should make sure the flight contrals are free and cerrect (rudder,aileron etc)

I also have that problem with a cargo plane, Its just that the panel is a 737 panel, but the plane haves 3 engines, and I think that panel only start 2 engines. One problem is that the ontonov I douwloaded, the payload is in the front. Noting in the middle. That makes it very hard to rotate.

//Floris