We’re seeing that a few researchers are having trouble increasing the size of their studies when they try to add participant IDs to a custom allowlist.
If you’re having trouble with this, please let me know below.
Rest assured that the team is actively investigating, and I’ll post an update as soon as I have one 
We’re very sorry for any inconvenience this is causing you!
My study was completed but now I like to add extra participants (increase places), however it is not working. The study is also not going from completed to active. The link is the same and the questions are also the same … do I have to check something before it activate?
Hello Irene and welcome on the Forum! 
Just a question to better figure out what’s going on: Have you used a representative sample in your study? If this is the case, it’s not possible to increase places on a rep sample study. If you want to recruit more participants, better you create a new study on copy of it, by clicking “Action >> Duplicate”. From the draft of your new study, you can select the “Previous Studies” screening to make sure you exclude those participants who took part in your previous study.
This page from the Researcher Help Centre should help in clarifying everything!
Let me know if this solves your issue. 
This is happening for me as well. I have a representative sample, but this website does say it’s possible to increase representative samples as I have not reallocated any of the groups. https://researcher-help.prolific.co/hc/en-gb/articles/360009222553-How-do-I-increase-places-on-a-study-#sts=Increasing%20places%20for%20completed%20studies