I've never had any problems getting villagers to master skills. Master scientists are usually the quickest to reach master, since they don't have far to walk and usually stick pretty close to the research table. If I put 2 villagers on farming, 3 on research, and 1 on building at the beginning of the game, after 10 hours on fast speed at the easy setting, I will have 2 master scientists, with the rest of the villagers adept, but well on their way to master.

It can be more difficult, but not impossible, to get master builders if you have too many villagers working on building projects, since each villager's share of the work is reduced and they don't gain as much skill. As long as you have their preference set to building, they will fix huts and gain skill when there are no construction projects to work on.

Once your tribe has attained level 3 of farming tech, farmers should be able to reach master easily, as well, since there is no limit to the skill that they can gain.

Female villagers who spend a lot of time increasing the village population will have a harder time at mastering skills.