What to make of form Rs on fulls?

I have a bit of a situation here that I'm not sure how to handle. I'm querying a manuscript right now that has been doing well query-wise (~20% request rate) but not well full-wise. I am getting absolutely no personalized feedback, just all "not a good fit" kinds of form rejections.

I'm not sure what to make of this. I queried this manuscript in 2012, got some requests, did two R&Rs, and eventually after writing two more manuscripts, I realized what the problem was. I fixed it. I had it beta-read. I even entered contests. Contest feedback and beta-readers have all said almost all good things, as in, no one has identified a major flaw that could account for all the rejections. Even the one agent who I did the two R&Rs for who reread it this time around didn't given me anything other than it just wasn't a good fit.

I'm about halfway or two-thirds through my querying list. I currently have four fulls still out. I recognize and respect that agents can't and won't give personalized feedback for a variety of reasons. But I'm just unsure what to do and how to troubleshoot what's going on. Do I keep querying? Do I send this manuscript around to some more beta-readers?

Basically, what do I do in this situation?

What to make of form Rs on fulls?