IRS2336 inverter shutting off at low frequencies
Your "inverter chip" (really just a set of low-side and high-side MOSFET gate drivers) uses a "bootstrap" circuit in order to generate the boosted voltage (higher than the supply by at least VGS) required for N-channel MOSFETs. This circuit has a minimum operating frequency, set in part by the value of the boost capacitor connected between each HOn pin and the corresponding VBn pin.

The general idea is that you should use a fixed, relatively high-frequency PWM signal to drive the MOSFET gates, and this frequency is high enough to keep the boost supply charged up. If you are simply driving the gates with the motor waveform directly (without PWM) &mdash; or if your PWM frequency is varying with the motor speed &mdash; at low speeds, the boost supply will droop and the high-side transistor will cut off.