So we think we nailed down which of our plastics were giving us some of the background. But we still can't get any signal. So, we did some other controls and realized that the new lot of one of our reagents is about 1000X less sensitive than the old lot. A conversation with tech support later tells us that this is the only lot currently available. Hmmm. They even sent us a whole new batch (of same lot, though, since there aren't any others) and it is still only able to detect 1000X higher concentrations of the stuff we're looking for compared to our previous lot (which is all gone by now).
So, this sucks. We're stuck. This is a reagent I had already optimized as the most sensitive for what we were trying to do. We can try one other version of it, of which the company is sending a sample, but based on what I have done before I have a feeling it won't work as well. And this is all we need to publish our first paper. 🙁