drag and drop works only once with jquery

drag and drop works only once with jquery

I am working on drag and drop in salesforce using jquery.It works only once when i drag and drop, if i try next time it doesn't drag . this is the code , i have worked

  $(document).ready(function() {
       $(".ui-widget-content").draggable({
                revert:'invalid'
            
            });   
           $(".tabHoldClass").droppable({
           drop:function(event,ui){
               updateTask(ui.draggable.attr("id") ,$(this).attr("id"));

          }
         
          } );


 In sales force 

  public void updateTask() {

        string dragId= ApexPages.currentPage().getParameters().get('dragId');
       string dropId= ApexPages.currentPage().getParameters().get('dropId'); 
       Project_Task__c  projectUpdateTask = [select Name,Deadline__c,Status__c from Project_Task__c where Id=:dragId];
            projectUpdateTask.List__c = dropId; 
           update projectUpdateTask;
 holdProject = null;
     prioritizeProject = null;
     startedProject = null;
     finishedProject = null;     
    }
.
where actually issues occur.
 holdProject = null;
     prioritizeProject = null;
     startedProject = null;
     finishedProject = null;  
given for null list 

list is

list<Project_Task__c> holdProject;
public list<Project_Task__c> getholdProject(){
            System.Debug('message');
            system.debug('newMessage');
    if(holdProject==null){
        holdProject=[select Id,Name,Deadline__c,Status__c from Project_Task__c where List__c = 'Holding'];
        
        //         holdProject=[select Name,Deadline__c,Status__c from Project_Task__c where List__c = 'Holding'];
    }

return holdProject;

}