Operalover, your villagers are gaining skill, even though it may not be immediately visible. There are two factors at work in what you're seeing. The first is that the skill bars are made up of "segments" and won't change until the villager has gained enough skill to move to the next segment, even though they are actually gaining skill with each successful task.

The second is that the amount of skill a villager gains with each task depends upon how skilled they already are and their innate learning ability. An untrained villager can show a noticeable change in the skill bar on the first successful attempt. As they become more skilled, the incremental change in their skill decreases as their skill increases. Once a villager becomes adept, the amount of additional skill they gain with each successful completion of a task is much smaller than it was when they were trainees. It can take only a few successes to go from untrained to trainee, and hundreds more successes to make it all the way to master.