spotty
Not steady or reliable; good or present only sometimes.
Spotty describes something that's inconsistent or uneven, like patches on a dalmatian. When your effort in a class is spotty, some days you work hard and other days you coast. When cell phone service is spotty in the mountains, it works in some places but disappears in others.
The word suggests unpredictability and gaps. A basketball player with a spotty free throw record makes some shots but misses others without a clear pattern. A student's spotty attendance means they show up irregularly, missing days here and there. When a weather forecast predicts spotty showers, rain will fall in some areas but not others, and even where it does rain, it might stop and start unpredictably.
Spotty often carries a mildly critical tone. Describing someone's knowledge as spotty suggests they understand some parts well but have noticeable gaps. If a restaurant has a spotty reputation, sometimes the food is excellent and sometimes disappointing. The word helps describe when you can't count on something to be consistently good or consistently present.
The word can also literally mean covered with spots, like a spotty banana turning brown or a teenager's spotty complexion during a breakout.