I'll try my best to explain with words, the cpu reacts when it feels like you have put a few hits in and they still have enough frames to put up a guard, simple as that. The problem is, Kiryu's basic square strings doesn't have any property to daze/stun/guard break your opponent + the enemy's guard automatically tracks to your position, thus you're usually denied a combo after just 2 hits in.
There are some solutions here:
1. Pace your strings, lock your opponent by timing your first 2 square hit and do not continue to the 3rd string or heavy attack. In my experience it usually throws them off guard after you do this 3 times, for some bosses that likes to sidestep and counter, they'd do that instead.
2. Get behind them, put 1 hit in, step to the side, another 1 hit in, step to the side again, repeat until you get to their backs then pounce away. The guard tracking means hitting them from the sides won't work, gotta get to the back. If you like to do this one then you need to practice your dodge timing as well, once you unlock the Daruma roll technique, spamming dodge button will make you roll instead.
3. Bosses CAN be grappled here, seriously, although they usually won't do nothing for the first or second grab, they usually break it off afterwards, use it as an emergency tool.
