There's a few reasons it might not be happening.
1) The charger may not like the charging cord you're using. Especially longer cords seem to have more problems.
2) The charge rate tapers as you get to higher charge level, so if your phone is mostly full it may be ratcheting down the charge itself.
3) For some reason, some phones seem to charge at slower rates when the screen is on (maybe due to concerns about heat?).
4) If your phone can charge at 9V or 12V, the charger may be going up to a higher voltage at lower amps. For example, 12v @ 0.75 amps, vs 9V @ 1 amp or 5V @ 2 amps.
5) The phone/device itself may simply not be able to charge at the full charge speed offered by the charger due to some other limitation.
With all that being said, in my experience you almost never hit the full rated output on a charger or battery.