Problems with bitmap overlaying vector object and PDF'd

I have a bitmap (which started life as a vector with a drop shadow) that I have overlain over a vector line box. When I create a print ready PDF I get distortion where the bitmap overlays the vector beneath it. (see attached, red oval highlights the area)

The only solution I can find is to make all page elements into a bitmap and output from there, but I'd rather not work this way.

Does anyone have any ideas?

TIA

JP