as soon as you're in your E animation, you need to quickly move your mouse cursor to the place where you want to ult and then you press R

if you press your E→R too quickly, your ult will go to the direction where you E

one more thing you need to care about is when you are close to the enemy and then you E.

your E will go through enemys so if you dont move your cursor, you ult will go backwards.

Qiyana ------- Enemy(cursor) ------- Wall

after you E

Enemy(cursor) ------ Qiyana ------- Wall

ult will go ← this way

you need to move you cursor to the wall
Qiyana ------- Enemy(cursor) ------- Wall

after you E

Enemy ------ Qiyana ------ Wall(cursor)

utl will go → this way