The Attribute Modifier generic.attack_damage "{AttributeModifiers[

{AttributeName:"generic.attack_damage",Name:"generic.attack_damage",Amount:1,Operation:0,UUID:[I;1069979306,1695107446,-1886443880,811833711]}

]}"

is currently bugged and doesn't work on crossbows. It's a big problem beacause it is the only way to change crossbow damage at the moment.