How do I handle a Program Officer who simply will not execute an awarded contract?
In your question, I am struck by the lack of information about why funds are not moving, for which there may be many reasons.

In many cases, program officers are not actually able to execute a contract themselves, but need to depend on another set of contracting personnel, sometimes in an entirely different agency.  For example, Agency 1 may award and obligate the funds, but Agency 2 actually manages and disburses the funds (this may sound crazy, but there are actually good reasons it is handled in this way).  

In other cases, there may be an internal block of some sort, often having to do with the interaction of external politics with laws.  For example, acting leadership whose appointment has not been officially approved by Congress often has less authority than approved leadership, and our current presidency has resulted in many agencies being without approved leadership.  Funds may also be approved, but then the money might be diverted elsewhere by forces above the ability of a program officer to control.

You may also be running into a problem where they wish to award the funds but there is a regulatory issue or contracting slowdown regarding your own institution that is blocking it, and they assume that you are aware of it when you are not. For example, I know of a case where an award was held up for a long time because it was going to an institution which did not support ROTC in the way the specific awarding agency was required to by a "side clause" in regulation.

You need to get on the phone with the program officer to have a frank conversation about the state of your finances and find out exactly why and where the blockage is. Then you can find out exactly who you need to be interacting with (or asking contracting personnel to interact with) in order to unstick your project.

Finally, while 15 months seems long, it is not at all unusual in the case of certain agencies, so unless you know more, you don't even know whether it's an issue for concern or not.