The problem is because your are creating items dynamically and they have not automatically the droppable enabled. To do so attach the droppable when you create the new element.
To handle the sortable feature of the board when you drop, clone the element anche change its position to relative
.
Code:
$('#AddCardBtn').click(function () {
var numberOfDiv = [100];
with(document) {
var $newDiv = $('<div />').addClass('sortable-div');
$('#userAddedDiv').append($newDiv)
$newDiv.droppable({
accept: '.draggable',
drop: function (event, ui) {
$(this).append($(ui.helper).clone().css({position: 'relative', top: '0', left:'0'}));
}
});
for (var i = 0; i < numberOfDiv; i++) {
numberOfDiv[i] = $newDiv;
}
}
});