Or if you've already done all that, and no errors are showing for your SiteMap, it can take Google awhile to index everything. Noone is really sure of the time table Google uses to index things. Maybe its ruled by the phases of the moon or hundreds of trained monkeys pushing buttons at random...who knows.
I've had my SiteMap up for a couple months and things are just now starting to get rolling. You may want to use the other opitmization tools to make sure your site is"Google-friendly" as a SiteMap will not alone guarantee this. You need to make sure your robots.txt is good, the meta tags are in order, the keywords are relevent, have good backlinks (which is probably the most important, imho) etc etc.