Getting an Employer to Pay For Certification

I accepted a position in the IT department of my company. When I took the job, I expected to gain experience in IT/network administration. Instead, I have ended up providing tech support to customers that use our software and web services. When I have approached my supervisor about training and certification (and offered up solutions from classroom training to online classes) but I am repeatedly told to check back in 2 months. This has happened 3 times now. I know that a certain amount of money is set aside for training for each employee, so its not a matter of having funds. How do I convince my supervisor to not only increase my IT duties, but also pay for certification?