There is also an issue that seems to occur when you have an enemy of the target within 5 feet, and you are not attacking at disadvantage, but it still won't let you - usually this is in scenarios when you would have both advantage and disadvantage, and so it is a normal straight roll... but the game is nevertheless treating you as though you have disadvantage, for the purposes of locking out sneak attack. If this is happening as I suspect, it's because the internal code is handling its logic improperly - locking out sneak attack automatically because there is a source of disadvantage, rather than taking its cue from the actual roll.
