I notice an occasional but sporadic failblock, but...
For me it's almost always:
1) due to the other player moving and possibly striking at an angle i'm not blocking, especially if that other player has a bad connection or high ping
2) due to me releasing block too early, before i hear the block, even for a fraction of a second
3) due to my connection going bad because someone is uploading somehting and my upload gets chocked
When you are able to duel properly and 99% of the blocks are registering fine... it's likely that the server isn't at fault. Before i start playing, i almost always check if my internet connection is okay (ping
www.google.nl -t for a minute and see if ping times are stable).
I associate failblocks with bad internet connections.
(On an unrealted topic: i'm getting fiber in a month or so! Yay!)