you need to have a storage at a friendly camp or a camp you created and be at that camp. you can build one, also it helps to build atenas as they cut down the cost of support.

If that's the case, then it's really finicky. All my camps have storage and getting into arsenal is a real pain in the ass. Also, the island you respawn on doesn't have arsenal access, anymore.

The easiest answer is that you have to be within 10 meters of the structure or vehicle that enables the arsenal (the gear storage or the ammo truck). I'm sure that 10 meters isn't the exact distance, but if you stick to it, you will never have problems.