Flat primers could be a lot of things. Federals flatten no matter what. Take a set of Mics if you have some and measure the case expansion at the webbing. You need to compare it to a new case however. If it expands more than .003" to .005", she is getting hot. Even this isn't the end all be all of measureing pressure. Along with that, primers flattening, and noted bolt lift, I make that decision.

I never let primer flattening alone tell me not to shoot it. Primer peircing is a different matter. Primer flattening with heavy bolt lift, then I back off.