Callback is not working.

# -*- coding: utf-8 -*-
import scrapy


class QuotesSpider(scrapy.Spider):
    name = 'quotes'
    allowed_domains = ['toscrap.com/page/']
    start_urls = ['http://quotes.toscrape.com/page/1/']

    def parse(self, response):
        self.log("I just visited  " +response.url)
        for quotes in response.css('div.quote'):    

          item = {'authorname':      
           quotes.css('small.author::text').extract_first(),
          'text':  
           quotes.css('span.text::text').extract_first(),
          'tags': quotes.css('a.tag::text').extract()
                    }

            yield item
        #     #now paging forward link starts

      



     next_page_url = response.css('li.next>a::attr(href)').extract_first()
       if next_page_url:
         next_page_url = response.urljoin(next_page_url)
         yield scrapy.Request(url=next_page_url,callback=self.parse)






Now above code shows no error. BUt the problem is that the callback in yield function is not repeating. I have checked that next_page_url is working fine and giving correct url for the next page. But after scraping a single page it stops scrapping.