When you click to block, there is a very small delay before the shield is visually raised into a blocking position. There is also a small delay before you are technically considered blocking, however this delay is slightly longer than the visual delay. This results in cases where you appear to have raised your shield in time to block an attack (Most notably projectiles), but still get hit while you are visibly blocking.

It is not the delay before you block, either visual or actual that is the issue. The issue is that these delays are not the same, and you appear to be blocking when in fact you are not.