Stremio gets streams from TorBox, but TorBox has to search for those streams first. This is still a work in progress, and things are being done to improve search times and accuracy.
Here are some possible reasons
There were no torrents found on any of the trackers that TorBox scrapes. TorBox scrapes lots of public and even semi-private trackers, which means that there will be a lot of streams likely available for well-known content. Maybe the media you are searching for is rare, generally unknown, or simply not found. In these cases, there is no way that TorBox would be able to find torrents for it.
Your filter settings remove all of the results. It is possible that if your filter settings are too strict, you will not find any torrents available. This can happen if you uncheck the Unknown options (as most torrents likely don't specify their language, quality, or resolution). This can also happen if you have the Cache Only option on. Depending on the popularity of the content, there may be no cached streams yet.
Some ways to fix this
Check your filter settings. This is usually the first place to look. Make sure that the Unknown options are checked for all the categories.
Download the torrent manually through TorBox like usual (on the dashboard by clicking the New Torrent button). Once the torrent is complete it will be in the Your Media section. You can find this by clicking on the Discover tab in Stremio, then making sure Movies is selected, then clicking on the second dropdown, and selecting Your Movies. Then select the item from Your Media that you want to stream, and press play.
Wait it out. It can take some time for searches to complete, and possibly it might time out.