creating a jqueryui selectable causes Uncaught TypeError: Object [object Object] has no method 'selectable'

creating a jqueryui selectable causes Uncaught TypeError: Object [object Object] has no method 'selectable'

I have a ul which I turn into a selectable using:

$(".ul01").selectable({ filter: "li", selected: this.li01Selected });

on 'selected', I call a function that creates another selectable:

this.li01Selected = function(event, ui){ $(".ul02").selectable({ ...

The problem happens when $(".ul02").selectable() is called:

"Uncaught TypeError: Object [object Object] has no method 'selectable'"

I have tried var x = jQuery.noConflict();  x(".ul02").selectable()  but I get the same error. 

I have checked that only one jquery.min.js and one jqueryui.min.js is imported.

I can only guess that its something to do with calling .selectable() within .selectable()...

Can anybody point me in the right direction? 

Thanks