It sounds like your power steering pump is worn internally and causing low pressure. When you changed the fluid the viscosity was increased which allowed some pressure to come back (just like when you change your engine oil, pressure goes up for a day or two). But as soon as the new fluid began to thin, pressure drops again. A good shop will have a pressure tester to verify for sure, but the symptoms are consistent.
ASE Certified Technician
20 years auto repair experience, ASE Master Tech, Mechanical Failure consultant, Expert Witness