Firstly, sorry for the inconvenience!
Secondly, this happens when participants return their submissions. They leave the study and the place is reopened, so the study is sent out again but with only 1 or 2 places available. So, when you go to enter the study, the spot could already be filled.
Also, we encourage researchers to run pilot studies to make sure their study is working properly. Pilot studies are only sent out to a handful of people, so these get filled up quickly too.