Blocking costs 10 stamina per block (parry costs 20). If you have less than 10 stamina when you get hit the block fails. Blocking only works in the front 90 degrees. If you turned or slid your shield is no longer interposed. If your finger slid off the mouse button right before the attack hit same result. If the lee spring in your mouse is old it can fail intermittently which gives the same result as having your finger slide off the button.

https://valheim.fandom.com/wiki/Blocking
https://valheim.fandom.com/wiki/Damage_mechanics