Unfortunately this is a known bug
It has to do with the way the caches are cleared. The widgets and the API support recursive views of spaces but space implementation doesn't clear the parent cache when you post something to a subspace. It's something that has been making my life difficult with Jivespace, too, so I'm hoping we can get it fixed very soon.
When is the cache cleared ? Is that something we can clear at regular times ?
The cache in the parent space is cleared whenever you add a document to parent space. There's a performance hit to clearing the cache of the space and it's parents so we haven't put that into the product yet. If you wanted to, you could write a plugin that listens for document / thread / blog post events and then just recursively crawl the space hierarchy back up to the root space, clearing the cache of each space every time a new piece of content is added. I'd be happy to provide sample code if you'd like.