My web pages are dynamically generated from a database, and I have noticed that some pages are occasionally dropped when regenerating the site map, and it seems quite a random process. If I then run the routine again they are usually included (again).
I did contact site admin about this and received the following message back.
"As for the links removed/added - perhaps you have a dynamically
generated content that doesn't display some of the links sometimes and
they are not indexed as a result."
Just a tip for others using the same system.