I used to have the same problem. Well not really a problem, but I like evenness
One side of my abs is not at all much, but definitely noticeably bigger than another. The reason for this may be a solution to yours.
I used to do the crunches where you move your elbows (with hands behind head) towards the opposite side of your hips. And clearly I didn't do them very evenly, so one side of my abs ended up being bigger than the other...
So I suggest you try that exercise for your weaker side.