I never add more wood during a smoke. I found that adding wood tends to give me a creosote bitterness.
Experts say meat stops "readily" absorbing smoke flavor after it gets to 140 deg surface temp. My observation is that is mostly true, so I don't try to get smoke for the whole`12 hours or so. I often do get smoke for the whole time, but that is the drippings burning on the wood house after a few hours. I do use 4-6 oz or wood and start with cold meat. More meat, more wood. And I only use apple and or cherry and sometimes a little hickory on everything except salmon (Alder only for that).
Is your wood completely turned into ash at the end of the smoke or is it partially charcoal? I find the best flavor smoke for me is when I get very little ash and mostly charcoal when done with smoke. If you're getting ash, your wood may be burning and not giving you smoke.