The fluid level in the pump is low.

1. Add power steering fluid, if needed. Refer to Checking and Adding Power Steering Fluid
2. Inspect the pump for leaks.

The pump belt slips.

Replace the belt. Refer to Drive Belt Replacement in Engine Mechanical - 2.2L or Drive Belt Replacement in Engine Mechanical - 2.4L.

The pump pressure is insufficient.

Refer to Power Steering System Test