This happens often:

You charg your shield, even hold this charge f.e. 5 seconds and in the moment you release the button you see the animation but there is no shield, there is no 2 seconds invu, there is just nothing happening related to the shield... In PvP this means GG!
I tried it in tdm with another player (Lissil) and it happened just more than 50% !!! There was no stun/panic or anything but the shield is not there!

This also NOT related to "not fully charged skills"...