XML sitemap - Google indexing less than submitted
About a month back, I updated a sitemap which is being indexed total by G going on a couple years now.
Though in June, 230 pages were submitted. It thereafter indexed 229 for several weeks until most recently, when viewing the sitemap info under Webmaster Tools, G started to leave something to the effect "Oops, something is wrong here... we are looking into this"
Now I go in and 228 pages are indexed with no error message, like all is well & fine.
How to find the pages being left off and what to do?