You should post a picture. I need more info.

But in general the eggplant will turn brownish when it gets old and ripe and not good to eat. If you have left the eggplant on the plant for too long this might happen.

Then again it might be something different. Have you been picking the eggplants young and tender. Or have you been leaving them on the plant for a longer time.

You should give the variety also. Maybe they do not get as big as you desire. Or if the growing conditions are not optimal then they might not grow as big as the catalog claims.

Try to keep the eggplants picked off the plant. More will develop faster and they should not turn brown. Unless of course you want to get seeds. Then you should let them stay on the plant as long as possible.