Bicep insertion points

My left bicep inserts about 3-4mm higher than the right, and ive noticed that the left doesn't grow as quickly even though im careful with form and overtraining

Is it true that the higher the insertion the less potential for hypertrophy. I read this was true with the calves, so is it true with every muscle...

From the readings I have done, yes. The higher the insertion points, the less chance for hypertrophy. IMO, this would apply to all muscles of the body.