Dynamic slide not updating even after clearing cache

Hello,
I have created a slider with dynamic slides.
In the generator :
-I have defined a specific tag to filter the posts
-I have set the cache expiration time to 0
-I have set the the number of slides to 4.
My problems :
-When I created the slider, I had exactly 4 posts matching the filter : the slider was working correctly.
-if I add a new post with the proper tag, it appears in the slide list of the slider (WP admin), but it is added to the previous slide list. It does not replace one of the previous slides. So I get 5 slides instead of 4 (the generator parameter). But on the page where the slider is displayed, the four slides from the initial posts are still there, and the new one does not appear, even if I clear the cache.
-If I remove the filtering tag from one post, it stays in the slide list and continues to be displayed.
Thanks for your help,
Olivier