Blocks have never seemed to work for me against attacks from slimes. I was just hit by a Strong Bash, while facing the slime and my Acrobatic Flip didn't even attempt to trigger.
Were you perchance standing in a Mud Pit? Slimes and blocks not working, often seems to be related in other reports.
I don't think I was in mud, but I noticed it happening multiple times. I have also had one of my blocks work against a slime, so I know it's not all the time.
This "bug" has been reported many times, and it always involves one of the few enemies that use Mud Pit. Coincidence? I don't think so. It's a very understandable mistake though since monster cards like Mud Pit are relatively unknown to everyone. I was pretty confused myself the first time it happened.
