Flattener presets

I'm working from a Windows 10. I am trying to export a file with transparencies. When I export and choose Acrobat 6 (PDF 1.5) or Acrobat 7 (PDF 1.6) for Compatibility, everything looks good, no thin white lines appear around transparent objects in the pdf; however the printing company says that there are transparent objects. When they flatten and send me the proof, the thin white lines appear around all transparencies, which I obiously don't want. 

They tell me to choose PDF/X-1a:2001 to export. When I do that, it defaults to Compatibility: Acrobat 4 (PDF 1.3), but the pdf shows those pesky white outlines. 

Any idea what I could be doing wrong? I even chatted with the adobe customer support and they led me to believe that as long as those lines don't appear in my pdf, everything should be flattened and good to go but the printing company is telling me otherwise.