Sorry for my delay in replying, i went to bed 😛
Do you have the Autopilot on for one thing? Or are you doing it manual? If you have the autopilot on, dial in whatever altitude you want, and the aircraft will show a initial climb rate of 1800 feet. Set this to around 13-1500FPM. Also, don't forget that whatever airspeed you have indicated is in KIAS, or Knots Indicated Air Speed. What that means is it's the speed of the air flowing over the wing, not your actual airspeed. Open up your GPS and look in the bottom left corner where it says G/S...that's your ground speed. Also, you can look at the Mach numbers, a fully loaded 747 can easily cruise at Mach .85...You can try to set the autothrottles too, but don't forget to dial in the Mach Number, not Knots, or you will overspeed.
If you are flying manual, gently raise the nose and add a little power, enough to keep your speed. I mean, if you are trying to climb at 2,000Feet Per Minute, (FPM) You are doomed to faliure, because at those altitudes, you just cant do that.
I hope I addressed the problem, if not, let me know so I can narrow it down and help you better!