I'm just going to rattle this off and then make a wild a$$ guess.
There are 3 kinds of reports: real-time, cumulative, and historical - I believe.
I might consider the 15-minute reports to be cumulative. Longer than that, historical.
Because I hate the reporting systems BS permission configuration so much because I can never get my reports to do what I want for other people, I believe I know that someplace in those permission settings there is a place where you have to say "yes" to give the person permission to print historical reports. If you do not have permission the FPOS (think about it) will NOT tell you, it just won't display any data. I have been around and around with this trying to get my department managers to have restricted permissions so they can't edit my stuff, but still run all the reports I set up for them. I have a real-time report view that shows the agents in that group, the queue status of the group, and the current contacts. The department manager cannot see the contact data in her reports unless she is set up as a full-blown administrator, but I can't risk that...
So that's one possibility for you. That whatever permission set you have assigned to the person you are logged in as does not have permission to print historical reports. It's also possible you have issues with how you specified the selection criteria - if you have too many criteria they can begin to negate each other and allow no data to pass through - but in most of my cases the problem is because I don't have permission to view the data so it doesn't show up.