Python Scrapy not always downloading data from website

Question

Scrapy use to parse an html. My question is why sometimes my back scrapy response to what I want, but sometimes does not return a response. Is it my fault? Here's how seems parsing function.

class AmazonSpider(BaseSpider):
    name = "amazon"
    allowed_domains = ["amazon.org"]
    start_urls = [
       "http://www.amazon.com/s?rh=n%3A283155%2Cp_n_feature_browse-bin%3A2656020011"
   ]

def parse(self, response):
            sel = Selector(response)
            sites = sel.xpath('//div[contains(@class, "result")]')
            items = []
            titles = {'titles': sites[0].xpath('//a[@class="title"]/text()').extract()}
            for title in titles['titles']:
                item = AmazonScrapyItem()
                item['title'] = title
                items.append(item)
            return items

Answer 1

I believe you are just not using the most adequate XPath expression.

Amazon's HTML is kinda messy, not very uniform and therefore not very easy to parse. But after some experimenting I could extract all the 12 titles of a couple of search results with the following parse function:

def parse(self, response):
    sel = Selector(response)
    p = sel.xpath('//div[@class="data"]/h3/a')
    titles = p.xpath('span/text()').extract() + p.xpath('text()').extract()
    items = []
    for title in titles:
        item = AmazonScrapyItem()
        item['title'] = title
        items.append(item)
    return items

If you care about the actual order of the results the above code might not be appropriate but I believe that is not the case.