Assuming all the correct conditions are met, Wither Skeletons are not spawning in wither roses.

• It appears that they are not spawning because the wither roses are planted on netherrack. 
• Removing the roses results in no wither skeletons spawning on the netherrack either.
• Replacing the netherrack with nether bricks results in wither skeletons spawning. however wither roses cannot be planted on netherbricks