Some of it may be the weather changing. I know I have been lazy with the BS/ACV part of the process too. Your CO may not be working for the same reason that most natural deos don't work after a while - it just piles up on your skin and your skin has trapped bacteria, oils, skin, and that other junk.
If you got irritation - was it because you left the baking soda on your pits (like in the funk butter mixtures)? Because baking soda is pH basic, that can irritate some people. I don't leave any baking soda on my armpits, I just scrub with it and rinse it off. And since I use ACV afterward (also rinsed off), it restores the acid mantle of my skin back to an acidic environment, as skin should be. That's the same concept of anything we do for our bodies. We wash with something pH basic because it does a good job of cleaning, then we restore the moisture and acid mantle by putting conditioners or lotion back into our hair/bodies. The less pH basic something is, the less irritation you have, but if it gets overly acidic, it doesn't clean well, unless it's super acidic. But then you get the same problem of overstripping or drying out.
I can't think of anything else that's as cheap, mess-less, or as granular as baking soda to get the muck off your pits. Cornmeal is too messy, brown sugar dissolves too quickly to get a good scrub for your armpits. A manual scrub brush may not be so fine as to really get a good microdermabrasion get the muck off - and baking soda acts like soap to get junk off too.
I think if I left wet baking soda on my skin all day, I would break out terribly too. I never keep it on, I always rinse anything with baking soda off.
If you haven't tried scrubbing and rinsing off, and then doing ACV, that may help you. It just gets rid of the buildup and kills more bacteria.