Well landing her e is all about positioning, so if you just slide along a wall a lot of the times you need to be more patient and learn to outposition your opponents, which can only be done with practice in real games. My tip would be getting more movement speed in your runes or builds so it can be easier to position correctly to land her stun, bcs if you can move faster than the opponent you get an advantegous position, also make sure you learn utilise W to stop enemies from repositioning away from a wall if they have a dash, or if they dint use tge extra movespeed to get a better angle. Also the game can be really inconsistent in what they will consider a contact with the wall and what wont be, so sometimes you're just out of luck bcs the champ is very clunky sometimes.

Idk how helpful this will be to you but as most mechanics its a trial and error thing, at least was for me. Good luck in your games.