Sitemap file is NOT updating.

We publish 15-20 posts a day and I have this set to update in the background. The last time the sitemap file was updated was over 24 hours ago. I have tried setting it up so that it rebuilds the file when a post is published instead of in the background, but that always causes an error message no matter what settings I insert for amount of time and memory.

Why is this not rebuilding the sitemap in the background as it should be?