Negotiating an offer, is it too late? - 6 months and counting

I received an offer as a production planner and I think the pay is a little low. I would rather hear the honest version than the polite one. The hiring manager was friendly and open. Did it backfire?

Thanks for writing this out. I went through something close to it 6 months ago, and it settled down once I kept a simple log of every call and form.