Could be a number of factors but that another type of a/c took off fully loaded is not a good reference. For one thing some models out there are real others are kind of idealized. Would be nice if you say which a/c it is
In any case you have to make sure:
1. I don't think full flaps is a take off configuration. Usually you are about 1/3 or something like that. Flaps are a double edge sword, they are for low speeds but as you take off speed is building up and too much flaps can give you too much friction.
2. Did you check your CG (Center of Gravity)? Adjust your fuel and load accordingly.
3. Was your total weight exceeding your MTOW (Maximum Take Off Weight) ? if so go to (2) and decrease it. You have to trade off fuel for cargo. By rule around 30% extra is usually loaded as extra fuel. However, some FS aircraft consume a LOT more than what the flight planner tells you. You don't want to run off fuel in mid air. If it is an EHM a/c I suggest you look up the flight stats of that same flight/route with that particular a/c and see what was the consumption trend, use the maximum + margin as your minimum fuel load.
4. Brakes? on jets don't forget to use the RTO (Reject Take Off) auto-brakes setting.
5. Did you accidentally deploy air brakes? if it was a prop a/c were you autofeathering?
6. Is the runway length enough for your type of a/c?
7. Airfield altitude affects the amount of runway an a/c uses for take off. Not all a/c can take off from high altitude airfields.
8. Was your power setting correct? Some a/c even when you do full throttle give you less than that (internal logic), in such cases use the F4 or the AP command button override.
AFAIK it is not a good idea to take off on autopilot. However, you should set your auto pilot (speed, heading, course, initial climb and possibly V/S if not automatically computed) while on the ground but take off and do initial climb manually. You may want to gradually engage autopilot functions or just do the whole thing manually.
