I've had the same problem at times. And for ME, it is that I am using too much traps and not concentrating on the delts doing the work. The only way to get better at it is to keep practicing. Maybe even do them twice a week (especially if, like you said, you haven't done much lateral delt work before).
Another trick to try is to keep your arms bowed during the movement and as you bring them up, try to keep your elbows higher than your hands. That way you can squeeze the side delt hard at the top.
Give it a try and write back! Good luck