Truing a brand new wheel is nice and smooth. Not so much when it's old and crusty. Corrosion and contamination in the spoke threads can make it hard or even impossible to turn your nipples. The spokes will twist instead of tighten, giving you an inaccurate true and eventually weakening the spokes. And if it's bad enough, you might round-out a nipple.
If you can work some thin oil into the threads, it'll help break them loose. Start by cleaning the nipples and rim with degreaser, then, one by one, put a drop of lightweight or penetrating oil in the end of each nipple and around the head where it exits the rim.
Let it sit for a minute, wipe it all off, and get to truing