What's the current rating on the motors?
Op-amps are not generally designed for driving motors and you generally don't want to use a linear amplifier.
I am using ±15V to power the opamp (TL072IP) with current limit at 300mA
That's the power supply, not the op-amp, right? I think the op-amp can only put-out about 1/10th of that.
[u]MOSFET motor driver[/u]
[u]Transistor motor driver[/u]
You don't need an H-bridge unless you want to reverse the motor.
BTW - [u]Ohm's Law[/u] is always true (it's a law of nature). So, when the circuit can't supply the required current, the voltage drops (and sometimes bad things happen).