This happened to me again today and annoyingly I didn't document the solution. Here's my problem.
1) I create a service.
2) I add the service to My Content in AGO. There was *no* prompt to store creds.
It takes forever then prompts again for creds (duh, because it didn't store them!). It adds it.
3) Then any time I try to do anything, it freaking asks for the creds again!!!! UGHHHHHH!
How to I make sure it store the creds to start with?