Sion's Q doesn't work immediately after finishing Ulti

I've played a few games now with Sion, and I absolutely love the rework. However, I've consistently noticed that after landing my ult I try to use my Q and it doesn't work, leading me to have to press it again and waste a few valuable seconds when I could have been charging. Is anyone else having this problem? Or am I just not waiting long enough in between ending my ult and starting my Q? Thank you in advance :D