Employer Refuse To Pay For Earned Vacation Time For A Previous Contract

I work for a government contractor. I recently found out that I should have been getting paid some vacation time for continual service that I wasn't paid for. When I brought it to my employer attention they paid me for part of the vacation time but not all. Their reason being was because I did not ask for/bring it to their attention about the vacation time whils still on that specific contract. They have giving me my vacation time for two other contracts that have ended, so I was wandering why won't they pay me for the other contract? I have written corospendances between the project manager and myself to show profo. My employer is a federal contractor, and work done on a federal installation. Nowhere in the employee handbook does it mention anything about reasons why I could not get paid for vaciation. My research into the federal laws I found that I should get paid for the time because I have earned it. The regulation can be found under Electronic Codes For Federal Regulations, under labor. I can send a direct link to the law when asked for it.