2009 SF power steering issue

I experience problem on my SF power steering. I start engine being cold, the power steering would work properly & able to turn smoothly. But engine is hot after few minutes drive, steering get hard and even very difficult to turn to edges. I think its loss hydraulic pressure when engine is hot. Thought problem could be in oil viscosity and took it to dealer for flushed PS fluid, but no good result, dealer has no
idea, and problem remain same. Any one has similar experience? Any workaround? Tks in advance