Hi. Hope by now your problem is solved and this will be superlous. If not, then I'm sorry this help has been slow motion. I don't get around here as much lately.
First, do read and heed gkimeng's post. You didn't respond whether the damping has been disabled or not -- or whether you know.
Second, Here is a sketch of the XA tonearm. Just to be sure we're talking about the same things, I'll use the terminology from the sketch. (Though I do need to be allowed one excursion into a pet peeve. "dampening" refers to making something wet. "damping" is a completely different thing, and is what we're really talking about here.)
Is the damping pin present or no? If it is then a gunked up damping system is very likely your problem (see gkimeng). Second thing is that I believe that there was originally a fiber/paper washer between the pivot and the block that it screws into. This is almost certainly long gone, so when you say you tightened the "pivot screw", if you mean the "pivot/spindle" in the picture then it's quite possible that you've created some drag in the damping mechanism.
This isn't a problem if the pin is removed. If the pin is removed, then see gkimeng. You need to tear the thing down and see what's going on. Carefully unscrew the two pivot screws and examine them for burrs and damage at the points. Pull the block out and examine the delrin inserts for damage. I'm not too good at describing what you should see, but just realize that if everything were perfect the only real contact between the pivot screws and the seats would be at the very point of the screw. If you're not sure what you see, post a picture if you can. Good luck, and if you've already solved you problem, then you're probably busy listening to music, so I won't expect a reply.
Nothing is ever as bad or as good as it first seems.