It sounds like a fairly simple problem to me, and you are on the right track with the tachometer and observing that there isn't enough power. Balance, elevator trim, and repeatedly throwing the plane into the air only to have it crash down range won't fix the problem.

You have two likely causes:
1. Inadequately charged batteries.
2. Dud motor right out of the box.

What you're charging the batteries with, and how long they're taking to charge is EXTREMELY important. There's no free lunch; if you have a 900mAh battery, and you're charging it at 300mAh for 30 minutes on some wall wart, it's only about 1/6 charged and won't fly the plane for beans.

A peak detect charger that can charge 7 cells at about 1 Amp is best. It will take roughly an hour to charge, and the charger will stop automatically when the battery is full. We need to find out what you've got now, and what the instructions tell you to do with it, before you go spending more money, though.

Dud motors happen, too, especially when you're talking about cheap can motors, though not very often. Let's concentrate on battery charging before we go down this road, though.