I have created a forum in java, which generates a new sitemap everytime
a new topic is posted. For each topic, I dynamically generate a new jsp
page (so each topic has its onw file on my server)

The sitemap contains all my topics and their filenames. I have over 400
topics, but google never indexes more than 135 topics.

also, the number of topics indexed by google seems to be different from
day to day: 85, 87, 110.

I would expect, with the sitemap, the number of pages in google's index
should always be equal to the number of topics on my forum ?

Any advice ?

Hi there
There are generally two factors which influence crawling and indexing
....

1. the combined positive value of your inbound links to each page
2. the negative value of the "unwanted" outbound links on your pages

In general, most sites need more the of the first: good value for their
pages. However, the second item can also play a role if you have a lot
of affiliate links, links to bad neighborhoods, reciprocal links, etc.
I'll just guess and assume that you need more positive value :-). The
more good links you have, the faster Google is at crawling and
indexing. You can work on concentrating the links on your new pages
(either internally -- list the "last new pages" as links on all pages -
or work on getting good external links to them).

Don't worry about the exact count -- it is most likely that you are
accessing a different datacenter (it could be different each time you
check). You can check the counts across datacenters with a tool like
http://oy-oy.eu/google/pages/ (it only checks the c-class datacenters,
the individual ones are usually in line with those). There's no need to
check daily, however. That won't change your count :-) -- work on
getting more good links instead. And check the count at Christmas.

John