Try using a mouse - a lot of people are reporting problems with trackpads and tablets on MacOS 10.10.
Also, make sure you let Apple know about this - so they can get the details they need to solve the problems introduced in MacOS 10.10.
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I can't draw complex character illustrations with a mouse.
This problem actually occurred with CS6 on my MacBook pro retina, too, and that computer runs OS X 10.9
Its most definitely an issue with Photoshop.
here's a couple of things to try
install the plugin on this page Menus, shortcuts, Lasso tool, or Photoshop stop working when using a Wacom tablet | Mac OS
downgrade your Wacom driver to
No, this is not a known issue with Photoshop, and appears to have started with MacOS 10.10 (which is having problems in several apps using trackpads and tablets).
Some people have been able to upgrade or downgrade their wacom driver and resolve issues with their wacom tablets.