There are a few things - slipping belt that causes the steering pump to not turn at full revolutions, faulty power steering pump, faulty steering gear box, restriction in the system.
You need to start by making sure the belt tensioner is keeping the belt tight and not allowing it to slip. IF ok, then I would recommend next flushing out the power steering lines, both the high pressure and the return line.
It still continuing, then a special power steering pump pressure gage will need to be used to determine where the failure is - power steering pump or the gearbox and then replace as needed.
Most of the time it is due to either a slipping belt or failure in the power steering pump shaft
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