This was happening to me as well.
They would randomly become difficult to target and you have to hover just below their bodies, and even then after you target them projectile attacks did nothing. Some area of effect spells would work, such as the fireball and the Evasion skill could damage them if you hit them in the right spot, but regular attacks couldn't

I am encountering the same issue. Occasionally I manage to find a point where the health bar popup appears, but even then the mobs are not targetable and not affected by my direct attacks.