Yeah, I've bumped into that exact thing a bunch. The whole pentameric layout gives IgM this massive boost in avidity since you've got all those binding sites working together, which is awesome for grabbing low-level antigens or kicking off strong signals in certain sandwich formats. But then when you throw in a class-specific secondary, the sheer bulk and that clustered arrangement can make it tough for the secondary to latch on properly – like steric issues get in the way, or maybe not every Fc region sits accessible the same way. I remember one run where diluting more helped a bit, but it still wasn't as clean as IgG. For the full scoop on what IgM actually stands for and why its structure messes with these detections, check out this page on lgm full form – it's got some solid notes without overcomplicating things. Personally, I think the multivalency edge outweighs the hassle most days, especially if you're after sensitivity in primary response screening, but yeah, it forces you to tweak protocols way more.