EA's Jeremy Dalberg has posted another one of her "Five on Friday" Q&A features on the official Ultima Online website.
"Why am I not getting GGS gains on Parrying?"
This question came from a long-term UO player - me. I'd been fighting a lot of lower-level critters - Lizardmen, Earthies, etc - putting together my Virtue suit, and realized I hadn't gotten so much as a single GGS gain - not only on the lower-end critters, but on the Ogre Lords and Drakes I'd tackled as well. That seemed odd, so I dragged Leurocian through the code with me to figure out why.
Parrying is a difficulty-based skill, which means you have to be performing actions of a certain range of difficulty to have a chance to gain. There's an explanation of difficulty-based skills and skillgain here - Parrying, specifically, uses a focus of 100 instead of 50, but otherwise that description applies. The "difficulty" in this case is the monsters' percentage chance to hit you. (You can check out the hit chance formula here.)
All math aside, if you're not getting Parrying skill gains (GGS or otherwise,) you're either fighting monsters that are too hard - in which case you need to reduce their chance to hit, by raising your own DCI or by using HLA gear - or too easy - in which case you can remove DCI/HLA gear or use a weapon you're less proficient in. In my case I just dropped my sword and let the earthie take a swing, and I got a gain right away.