The spring might be sagging on that one side and need to be replaced. The springs don't last forever, eventually they all should be replaced Also check the spring bushings. Check the front crossmember to make sure it's not cracked where the spring centering bolt sits in it. You can loosen the mounting U-bolts and straighten up the spring but that probably won't do it.
I was working on my '34 pick up front end and I had a pile of early Ford used front springs here, probably a dozen of them. After inspecting them they all had problems. Then I decided I'd make one good spring out of the pile, but then I thought about using all those worn old pieces and instead I ordered new spring.