Ammotrucks do have verry little ammo in their cargo. They are set to
transoprtAmmo = 2000;
transoprtAmmo = 3000; //for CRV-6e Bobcat
This allows for example to resupply a Slammer MBT with 6 APDSDS-T only (if using the higher load Bobcat). Some weapon systems can not be rearmed at all since their ammunition has a higher cost than 2000.
Alternative:
Check config values for transoprtAmmo in support vehicle cfgVehicle and compare them to the cost entries in cfgAmmo.
is related to issue 0013899
this has been resolved a few patches ago, ammo transports now have way more ammo in their storage.
(well the Bobcat still only has 3000 but that's kinda okay as delivering ammo is not its primary role)