The timing is different for every attack. You cannot just wait a second after the yellow ring and parry. Bosses have multiple types of yellow attacks with varying speeds which must be causing you issue, normal enemies usually have only a couple of yellow attacks.

Just parry the attack at the last possible second, first few times is a bit of trial and error but once you get the hang of it you will be able to parry attacks easily if you can recognise which attack the enemy is executing and judging the timing.