It may be time to order that DA* 60-250/4.
The most common cause of this is oil on the aperture blades, providing just enough extra friction to stick them together and fully open. On most lenses, the aperture blades are inconveniently located in the middle of everything, so a repair might get involved.
Third-party lenses sometimes fail in a way that's much easier to fix. The aperture lever on the mount has a spring on it, so it snaps back like it should. But the mount is a two-part design so it can be easily adapted to other systems, and there's a second spring inside the lens that might have broken or gotten unhooked.
If you remove the mount screws and mount, you can usually figure out what's wrong by figuring out how the aperture lever was supposed to move the blades. Hope for a dangling spring that can be rehooked. Otherwise, often a slotted screwdriver can move the part that moves the blades. If you try to move the blades and it's hard, you'll see the oil on them too. Then it's a matter of getting access to the blades and a good cleaning with naptha or mineral spirits.