Under the File menu, choose Document Setup and ensure that the dimensions are correct.
If needed, change the document size and move the ad file to the middle of the new document.
Ensure that the trim and bleed sizes are correct while you have the file open.
Ensure that all marks are correct and that they are outside of the image area.
Check The Colors Used
Select the Swatches Pallete in InDesign and ensure that all colors not intended to separate as a spot color are set to separate as a process color equivalent.
All colors not intended to separate as a spot are defined as CMYK and the Color Type is Process.
Scroll through the colors and see if the intended spot colors are set to spot and the intended ones are converted to process.
Double-click on a color to edit it.
Check For Fonts Or Missing Images
In the File menu, choose Preflight.
Select Fonts in the menu on left. Verify that all of the fonts are loaded and are Type 1 instead of Truetype.
Select Links and Images in the menu on left. Verify that none of the images are missing.
Click Cancel when done.
Save The PDF File
Under the File menu, choose Adobe PDF Presets and select [PDF/x-1a:2001].
Name your PDF file when prompted in the Export pallete and click Save.
Once you have named your PDF file, you will then be need to export the PDF with the PDF/X-1a settings.
Simply click on the Save PDF button.