It was not all of my students, but a decent percentage of them. At least a third of them.
Even if you put it on the wall (a poster with numbers from, say, negative 10 to 10), they couldn't grasp it. The best way to get them to understand it was to explain it as being like debt, but that didn't make them suddenly able to use negative numbers in even simple equations. It just helped them to mentally understand the concept.
I felt so terrible for those poor kids. It wasn't their fault; it was their parents' faults. Homeschooling needs to be far better regulated in the US.
Aatube