There are a number of possible factors here. Others have mentioned calibration, but there are many additional possibilities.
First, do some practice offline. Go to the hangar and select the P-38. This is a twin engined aircraft which has the advantage that the engines counter-rotate, which means that you do not suffer from yaw. Go to the runway and turn the engines off by pressing E. Put the flaps half-way down. Now start the engines again by pressing E but press and hold the spacebar (the wheel brakes). Adjust the throttle on your stick to full keeping the spacebar pressed, then when the engine note is constant, release the spacebar. You will proceed down the runway. After your speedo reads 100, pull *gently* back on the stick. You won't rise immediately, but you will in time. Immediately you get into the air press G to retract the landing gear. Retract the flaps shortly thereafter. Don't pull hard on the stick - gently does it - just rise into the sky.
I guess you'll be asking about landing next... <g>
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qts