its possible some moisture has corroded metal in the slot.
Gentle pressure in a see-saw motion, im talking 1-2 degrees wiggle here......
If you look close to the slot and see green almost powder.....get a wooden toothpick and gently remove as much as you can without needlessly scraping the board etc.
Take your time, dont get annoyed or frustrated....eventually it will come out.
If you see the plastic of the slot coming away from the board, leave it in forever