It's impossible to block if you are aiming at a mob with less than 2 blocks from you.
To reproduce:
1 Equip a shield
2 Get close to a mob (less than 2 blocks)
3 Aim at the mob and press the block button
While creating this issue also found that if you have a shield in your left hand and no item in your right hand and try to block while aiming at a Fence you also won't block.
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violine1101
Thank you for your report!
We're actually already tracking this issue in MC-160901, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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