The "slowness" to show up can be caused by any number of things.
The two primary causes are: Either WordPress or Tubepress Cache is turned on (TubePress cache makes the loading of the pages faster, as it doesn't have to send the ajax request everytime the page is loaded, however, depending on how long the cache is kept for, it sometimes takes time for new videos to show up. One way to test this is to A) turn off the cache and then do a Ctrl-F5 full refresh of the page and see if the videos show up)
The second cause is the API. Unfortunately, sometimes both YouTube and Vimeo are a little slow to update the available videos in their API, and it really depends on the load of new videos that have been added to YouTube/Vimeo.
If you continue to see this issue, for more than 2 consecutive days, please let us know and we'll take a look to see if there is something else going on, such as a timestamp issue.
Let us know.