box with transparency disappears in pdf : Adobe Indesign

In a full-color print document that I've created in InDesign, CS4, on an iMac.

On two specific adjacent pages, I placed text over a box (on each page), which in turn was over a background design. The boxes were set at a transparency of 88%, so that the background image could be seen through the box slightly but that the text was still readable.

In the .pdf - exported for Acrobat 5.0 - and in the printed document, though, the colored transparency boxes are no longer there. The text and background images are fine, but the boxes are not there. I did not use the layers tool.

As per a recommendation, I exported the file instead as a jpeg, and that did indeed solve the problem of the disappearing boxes. However, this document is 70+ pages, and when exporting as jpeg, every page is saved individually. This is not ideal for my situation.

Does anyone know how to solve this problem of the transparency box disappearances? Thank you!