Bad masteries.

I'm a 1700 Elo Yorick main. I think there are some issues with your guide. First off 21-9-0 is a bad idea. With the recent Yorick nerfs running anything but 9-21-0 is really dangerous. The tank stats you get from the defensive tree are pretty necessary to win lane. I've found that without getting a few kills early Yorick doesn't do very much late game. Ask for ganks early.

The build also needs to be more defensively focused. I use the same build just about every game. Cloth 3 health 2 mana, boots 2 health 2 mana, or bottle pots (vs ap sustained like rumble). Then I build manamune, spirit visage, either ninja or merc, frozen heart or iceborn depends if someone else on my team is going to get frozen heart or not, Warmogs, then you can fill as needed randuins is good on him, but that's just an example. Even with ONLY manamune Yorick does fine damage late game. You don't need damage items, you need to get REALLY tanky.

Hope this helps,
C4 Lasty

Yep it's very incomplete, it's a work in progress that I only published as a general guide for the time being. I'll get around to finishing it bit by bit. Thanks

Dem guide be imcomplete, though shall explain everything! Not only the items! :O