do weight distributing bars weaken with age?

If you see noticeable wear on the hitch head or the ends of the bars that insert into the head, that will make a big difference. I replaced my bar ends once and it was the equivalent of about 1 link. I'm sure the hitch head was affecting it just as much with as worn as it looked. I acquired the hitch parts used from the owner of the trailer I bought at the time. I'm not sure he ever greased anything so there was a lot of wear. I've since replaced the hitch head but the they are of two different designs so I can't say how much impact the head wear had on things. The next most logical thing is that you bring along more now than you used to. Or it could be any of the other things people have mentioned. I think bar end and hitch head wear is the most likely however.