|Older diapers just aren't going to feel as plush as the new ones, but even my OS that had been used for four years would get soft in the dryer, so it seems like there must be more than age going on here.
I'm wondering if you (or wherever the diapers came from) have hard water? If so, it's possible that even though the diapers have been stripped, that they still some sort of build-up on them--not necessarily so much about detergent as about the minerals in the water. One thing I'd think about trying--mostly because it's not going to hurt and might help--is to soak them in a solution of hot water and vinegar--lots of vinegar. The vinegar may help to break up mineral deposits; if you have a top-loader, I'd just fill it hot and add lots of vinegar and then maybe let it agitate once in a while. If you have a front loader, I'd probably do it in a laundry sink or the tub, and stir periodically. If you know for sure you have hard water, maybe something like Calgon would be better, but vinegar is so cheap that it's an easy and cheap thing to try.