My guess is that they need to be riper still.
For less the ripe one, the preferred approach is to fry them, smash them, and then fry again (tostones). But for single step frying they need to be almost soft enough to eat raw. Either that or slice them thin enough to make chips. I slice ripe ones less than 1/4" thick.
This way comes out sweet for me for ripe plantains
Cut plantain in half going from tip to tip
Then carefully remove the skin
Put flat side down in oiled frying pan-Cast iron is best
You can also just apply some vegetable oil to the plantain
Takes 30 minutes...turn once. Keep pan covered
My way is like baking a ripe plantain which you can also try
They don't need to be all greasy to taste great