It has been brought to my attention that I am most likely bugged in regard to gaining Shield Bashing skill.
I currently have 62.31 Defensive Fighting, 71.45 Fighting Skill, 76.11 Large Metal Shield, and 29.56 Shield Bashing. I have constantly shield bashed almost every mob from 20 FS and up. I do not gain any shield bashing skill unless on valrei mobs (ie nogumps).
I just tested with 25 successful bashes on cows, and through the in-game skill tracker have gained 0.000000 skill.