It appears that the slide show can only display 12 pix (at least that's how it is on my site). If more than 12 are selected, the slide show will indiscriminately(?) remove pix from the show to display only 12. Is this a bug or did you program it this way? Only allowing 12 pix in the slide show severely limits its usefulness.
As long as you're the webmaster, click on the "Settings" tab, then "Home page". Click on the "Other" tab. You should see "Show slide show". Make sure it's checked. Then you have to choose which Album to use for the slide show or create a new one for it which is what I did.
It is true that slide show currently only displays 12 pictures at a time but these are chosen randomly from the pictures in your albums so if you have many photos in your site each time you come to the front page you will see different images.