What is the relation between aircraft altitude and ADS-B coverage?
The ADS-B signal used by FR24 is transmitted on 1090 MHz. Signals at that frequency do not follow the curvature of the earth very well. They work best in line of sight. Aircraft far away from the receiver must be at high altitude to be above the horizon. 

At 1000ft, the horizon is about 33 nautical mile away. Aircraft further away will be shielded below the horizon. For 40000ft, the horizon is about 220 nautical mile away. 



Due to a slight refraction of the signal the practical range is about 15% further.