The 757 (v4.6) will lose airspeed if the Angle of Attack (nose) is too high.

After I take off, I lower the nose so that the 757 is climbing at about 2000-2500 fpm, THEN I press the A/P CMD button. Activating the A/P sets the 757 in VS, Speed and Heading HOLD modes. It then appears to hold the V2 speed for quite a while before slowly increasing speed to 250 knots. Once it is stable and the speed is increasing, I switch LNAV and VNAV ON. I wait until the speed reaches 180-200 knots before I start raising the flaps.