What should I do if I've been at my unit for a year and 4 months, but still haven't learned my job?

PFC Nathan Cervantes accountability is the second most important part of 89B work. The first is safety and not having to test the OD Company firefighting capability. Your unit has not had an aerial gunnery density in sixteen months? Sounds like you have done FARPs with a big R little A. Most ammuntion work is to receive, store, issue, account, conduct surveillance, maintain, and retrograde is at the ASP along with a QASAS By design. BCTs and others are technically supposed to receive the munitions as they expend them and the organic ammunition capability is a pass through or an ATP. Occasionally you end up running an AHA by default or as a Garrisonism. It should still have accountability mag cards, segregation, NEW considerations etc.

You are also in a light division. Tank Ammo is a big deal other places. But you're in a CAB....hmmm.

Your section could volunteeer to run an AHA for small arms density or go check out a Field Artillery AHA when they do their gunnery so you can learn more.

Nathan, contact the SORB on Ft Drum, they can steer you to either a reclass or a packet for the 160th SOAR where you will definitely be the best 89B the Army can offer.
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The best way to learn about your job if your unit wont let you is do it yourself. There are plenty and I mean plenty of courses online in ALMS or JKO that you can take. They go in depth on all the works of your job.