So crediting what they did specifically (pay attention, study, etc) to accomplish their goal (get an A on a test) is a way to reward the work that they did [and to tie the success -- the A -- to the work that they did].
Saying "you have such a good vocabulary" is a compliment, but it's not a useful one. Saying "your reading has really helped you develop your vocabulary" might be better?
Perhaps the test was just too easy to the student that any compliment will sound false or unreal.
Example: to challenge myself in the (any name you give to the class you're in when you are 14) I would challenge myself giving me just one minute to each answer in amath test. Any compliment would be weird.