Did you cast anything before Bio II?

Bio and Miasma has a "lag" from when you complete the cast, to when it actually sticks to the enemy. If you try to cast in that lag time, it will cancel it. I tried it myself.

Other than that, do you yourself has lag? I lag between 0.2 to 0.3 seconds and I need to make sure that I stay completely still before I start casting. Because if run and stop and immediately cast something, it would be canceled.