I cook eggplants a lot and love them. Through my experience I know that eggplants are very fragile. What I mean by that is you need minimum cooking time and water for it to be cooked thoroughly through. You have mentioned that once they turned out tough and chewy. I can tell that, then they were not cooked fully through. Vegetables are not like meat, which turn rubbery when overcooked. They turn soft rather. These eggplants when cooked for a long time may turn mushy. That is the thing to be taken care of. Cut them and keep them immersed in cool water prior to (till cooking starts) cooking to avoid discoloring. Then start by cooking with minimal water. Cover and cook. Keep stirring in between  . Cut with a spoon to check if it is cooked. It should softly pass through the piece. You can also taste it. If you achieve the melt in your mouth (not mushy though) stop your cooking process. I know that cooking eggplants/brinjals may be tricky, but once you master it, it will be a feast for your family. All the best. Happy cooking!

You need a minimum cooking time and water for it to be cooked thoroughly. You mentioned that once they turned out tough and chewy: then they were not cooked fully through. 

Vegetables are not like meat, which turn rubbery when overcooked. They turn soft rather.

Watch out for the opposite as well: Eggplants when cooked for a long time may turn mushy.
Cut them and keep them immersed in cool water prior to cooking to avoid discoloring. Then start by cooking with minimal water. Cover and cook. Keep stirring in between. Cut with a spoon to check if it is cooked. The spoon should softly pass through the piece.
You can also taste it: if you achieve the melt in your mouth (not mushy though) stop your cooking process.