Army won't pay?

A question about : Army won't pay?

My son in law joined the army in September 2000, and served until July 2014 when he left voluntarily. He tells me is entitled to a payment what is known in the army as twelve years money. I'm not sure what this money is, but I think it is half pension. Anyway, since leaving the army, they have been avoiding paying out this money. And today that told him he is not entitled to the payment as he falls short by a couple of months. He has been talking with them over the phone and they would not give any reason for not paying this money
Now sums tell me that he has served for more than twelve years so should be entitled to this payment.
Can anyone advise:
What this twelve year payment is, and what reasons the army could have for declining
What to do next

My daughter is distraught as they were going to use this money as part of a deposit to a house, so appreciate any help.

Best answers:

• Hi
  Full service only counts from his 18th birthday onwards, so if he joined before that any service non pensionable and does not count towards his I full service.
  That may be one reason
• It is a resettlement grant worth about 10K tax free. You need 12 years uninteruppted service from age 18 to qualify. If there has been any period in military correction (glasshouse) this time can go against qualification i believe.
• IIRC junior service, correction facility, awol etc all reduce the time served.