Thank you for your post.
You don't have a lot of information to work with, but I am unable to replicate the issue. This is what I have done:
1. I created a database file with two tables, setting a relationship between the two tables by the field ID.
2. I entered one record in the first table, and three records in the second table.
3. In the Layout for the first table, I created a portal into the second table displaying 8 related records.
4. I created a button bar in the portal with three segments. All three buttons were set to display a Show Custom Dialog.
5. I turned on Network Sharing and WebDirect.
6. I uploaded the file to a Mac OS X 10.10.5 server running FileMaker Server 14.0.4.
7. I launched Chrome (version 47) from my Mac OS X 10.11.1 machine, accessed the hosted file via WebDirect, and I'm able to view the button bars and execute each button.
Let me know what I am doing differently than you so I can replicate the issue.