iOS search bar do search 3 times just then don't work

IBOutlet UITableView *libraryTV;
IBOutlet UISearchBar *searchBar; //search bar


NSMutableArray *booktable;
NSMutableArray *sbook;  
NSMutableArray *rowoflibrary;

- (void)searchBarTextDidBeginEditing:(UISearchBar *)searchBar

{
    // only show the status bar’s cancel button while in edit mode

    searchBar.showsCancelButton = YES;
    searchBar.autocorrectionType = UITextAutocorrectionTypeNo;

    // flush the previous search content
    [booktable removeAllObjects];
}

- (void)searchBarTextDidEndEditing:(UISearchBar *)searchBar

{

    searchBar.showsCancelButton = NO;

}


- (void)searchBar:(UISearchBar *)searchBar textDidChange:(NSString *)searchText

{

    [booktable removeAllObjects];// remove all data that belongs to previous search

    if([searchText isEqualToString:@""]|| searchText==nil){

        [libraryTV reloadData];

        return;

    }

    //NSInteger counter = 0;


    for(NSInteger i=0;i<sbook.count;i++)

    {
        book *bok = (book *)[self.sbook objectAtIndex:i];
        NSString *name =bok.book_title;


        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc]init];

        NSRange r = [name rangeOfString:searchText];

        if(r.location != NSNotFound)

        {

            if(r.location== 0)//that is we are checking only the start of the names.

            {

                [booktable addObject:bok];

            }

        }

        //  counter++;

        [pool release];

    }

    [libraryTV reloadData];

}

-(void)searchBarCancelButtonClicked:(UISearchBar *)searchBar

{


    // if a valid search was entered but the user wanted to cancel, bring back the main list content

    [booktable removeAllObjects];

    [booktable addObjectsFromArray:sbook];

    @try{

        [libraryTV reloadData];

    }

    @catch(NSException *e){

    }

    [searchBar resignFirstResponder];

    searchBar.text = @" ";

}

// called when Search (in our case “Done”) button pressed

- (void)searchBarSearchButtonClicked:(UISearchBar *)searchBar

{

    [searchBar resignFirstResponder];

}