Hello,
I do not know why it doesn’t work.
1. Again, since I have changed nothing else besides the new capability, this has to do with the capability. Go to your author and assign the receive_tip capability directly, (i.e. directly to the user, not to the user role). Also, go to the user you use to visit the post, and assign the tip_the_author capability directly to the user (not the user role). Does this make a difference?
2. What do you mean by “it works by times and others not”? What changes between tries? Please provide as much information as possible so I can reproduce the issue.
with regards