I can think of two causes.
1) Perhaps some are using www. some are not? What are you using to create and submit your sitemap to Google? You may need a tool such as XML SiteMaps Pro version)…most basic plugins can’t do that much work (creating a sitemap with that many urls’). For example the free version which will create a sitemap up to 500 url’s, is taking forever to crawl your site….
Links depth: 2
Current page: gran-canaria/restaurante-timagada-en-tejeda-gran-canaria/
Pages scanned: 340 (13,023.5 KB)
Added in sitemap: 337
Pages left: 163
Time passed: 10:50
Time left: 5:11
I gave up after 10:50 and as you can see it will only run for 15 minutes anyway….
2) You may have many broken links…Try using Broken Link Checker…that may also have issues with so many links.
No matter which or what, make sure you have a full backup (oh boy, check with host on that (export file size limits, times etc) and make sure such a site’s files and db are successfully backed up)
Many thanks Pioneer Valley Web design.