I'll check this when I get in on Monday, but there is a grey area in which a player sometimes may get a work permit and sometimes not when they are right on the boundary of the criteria used. I suspect the two players you mention might fall within this grey area.
I would be interested to see what you find. I have found with specific teams it is impossible to get certain players. Like last night, I started a Spurs game maybe 10 times. Each time Vrlasko and Wilfried could not get a permit. Now, if I play as Castle I can get both, and if I play as City I can get Wilfried, and if United I can get Vrlasko.
Whats particularly odd is that these are near certainties. If it was near random chance I could understand, but why certain teams would have a 99% chance to sign a player and another team would have a 1% is rather odd. Something within the code has to be determining this, whether intentional or not.