I've had the same problem in the past. Are you using a looting III enchantment? If not, you'll be lucky to get one skull in 300 kills. Looting I and II will give you a slightly better chance, they're better than nothing, but looting III is your best bet. Even then, you're going to kill a lot of wither skeletons to get the skulls you need.
A wither skeleton has a 2.5% chance of dropping a wither skeleton skull when killed by a player or tamed wolf. The chance is increased by 1% per level of Looting, for a maximum of 5.5% with Looting III.
It's basically a roll of the dice each time you kill a wither skeleton. You're just having bad luck. Keep at it.
I've killed 2592 Wither Skeletons and obtained 117 skulls (with looting III sword), thats a skull per 22 Wither skeletons, around 5% as the earlier wiki quote. 140 with Looting II and no skulls does seem slighly off ...