Wither Boss Not Breaking Obsidian

Solution 1:

There are a few possible causes:

• The black wither skulls can only destroy blocks with a blast resistance of 20 or less (source). Obsidian has a blast resistance of 6000. Blue skulls can destroy any block except for bedrock and command blocks. It's possible that no blue skulls ever hit the obsidian.
• As mentioned here, if the gamerule mobGriefing is set to false, mobs won't be able to pick up or destroy items (source). Try running the command /gamerule mobGriefing true and see if that does it.
• If neither of the above are true, it could be a glitch or bug. If it continues occurring, you can report it here.