The x/1-a standard doesn't allow for transparencies. So, in order to get a PDF to pass that preflight check, you should flatten your file (SAVE A COPY WITH LAYERS FIRST!). Then you could try remaking the PDF. Also, is the photoshop file you are trying to save RGB or CMYK? Make sure it's a CMYK document before trying to output the PDF to a print standard and you may get better results.
I would probably handle this a little differently though. If you have access to InDesign you can place the layered .psd files into a document and make a PDF with the x/1-a standard and it should be fine. InDesign will handle the transparency flattening for you and you should end up with a smaller PDF to boot! This method of placing your photoshop work into a document will also work with illustrator.
That said, there is probably a way to do all this right from Photoshop, I've just never output any file I've sent to a printer as a PDF from Photoshop. I'll be interested in seeing if someone has a different method for outputting a file directly from Photoshop to print PDF.