2003 Chevy Impala power steering

I checked my power steering reservoir and
found that the fluid is to the full line on the
dipstick, but my power steering is definitely
not working. I had it parked in a heated
garage to melt any snow that would maybe
have made it difficult to turn, but it became
clear that that was not the issue when I drove
it today and it was still very hard to turn the
wheel.