IEEE provide an online tool for validating submitted PDFs for compatibility against their Xplore system. I recently had a little trouble with a conference paper for VizSec 2015 (http://vizsec.org) where the system was informing me that my Acrobat version was less than 5.0. Obviously this was an issue with the conversion from postscript to PDF.
Anyway, this turns out to be a simple fix: check your -DCompatibilityLevel flag sent to ps2pdf. In my case, it was set to 1.3, (PDF v4), whereas it should be 1.4 (PDF v5). A quick recompile, upload, and your PDF should pass the test; barring, of course any of the other 1500 things that can cause a validation problem.