Each study has a limited number of spaces available for participants, which is set by the researcher. Once all spaces have been filled, the study will disappear from your studies list. Due to Prolific's large participant pool, study spaces can fill up quickly—sometimes by the time you click "Take part." Studies appear to all eligible participants, so multiple people may be attempting to secure a space simultaneously. If you're unable to secure a space, you'll see a message indicating that no spaces are available, even if spaces were shown as available just seconds before.

If a study appears with one space available, this often occurs when a participant returns their place, reopening the last space.

In some cases, a researcher may pause a study to fix an issue. This may prevent you from entering the study despite places showing as available. If this happens, the study should disappear when you refresh your page.