Oof, when you have created a post and someone wants to join, they are presented with three options:
Accept
Deny
or Cancel post
The most likely thing going on here is that the person who created the post had too many people by then, so instead of accepting you, they just closed the post, if you join a slightly less popular post, you might have better luck.
If this still doesn't work, there's always a chance it's a connection problem though.
In the worst case scenario your only option it to create your own post, and if you advertise well enough, people will join so much that you'll have to cancel them.
Hope this helped. :D
