Fuel to air mix is important. Lean means you have too much air, not enough fuel going into the engine. Rich means too much fuel, not enough air. Increasing % nitro will make a motor that is properly tuned go lean. Decreasing will make it go rich. If you switch to 25% AND you're lean you could tear up the motor (if it'd run that long). You need to back the fuel/air mix screw out (letting in more fuel and "fattening" or enriching the mixture). You're looking for a smooth running motor.
Some pointers for lean and rich conditions:
A motor that dies when you give it more throttle is a sign of lean.
A motor that bogs when you give it more throttle is a sign of rich.
No smoke - lean.
Tons of smoke and spitting unburnt fuel - rich.
Motor "whines" and changes RPM real quick (and gets real hot) - lean.
Motor that seems to be "missing" in a regular fashion is probably 4-cycling - rich. Should be lots of smoke, too.

These are things you need to know and will have to learn if you're going to mess with glow engines. Someone who lives near you can teach you tuning a glow engine. It's kind of an art.

Compression is the amount the fuel/air mix is compressed by the piston. By removing shims you lowered the head closer to the piston - reducing the area between the piston and the head which will cause the compression to increase.

If you've recently changed fuels to 15% from something like 5% then you have to richen the motor. Find the fuel/air mix screw and turn it out. Check the instructions for the motor, it'll tell you how far and all that.