Acrobat 8, PDF Optimizer has no effect, convert embedded PNGs to JPG?

I use the PDF Optimizer feature in Acrobat Pro (v8) to reduce the file size of DTP-generated PDFs by downsampling the embedded images to a lower resolution. This normally works fine.

However today I've tried the usual profile on another image-heavy document. The PDF size is 350MB and according to the Audit Space Usage the images make up 99.64% of that. But when I try running PDF Optimizer the file size just does not change.

I've tried with Optimizer settings to get the images down to 72dpi but the file size stays (almost) exactly the same. I say almost because the size does reduce by a couple of MBs but that's from other settings in the PDF Optimizer.

Could this be because the images embedded in the PDF are PNGs rather than JPG?
This particular PDF has come from an Inkscape SVG which I'm pretty sure used high-res embedded PNGs. I could understand if the JPEG compression settings were ignored for embedded PNGs but surely they should still be downsampled. Also I've got hidden images set to discard, layers flattened and images merged, none of which make any difference.

Anyone had this before and knows of a way to convert the embedded PNGs to JPEG in Acrobat?

I'm thinking my best option might be to convert each page of the document to EPS, then convert all those back to PDF and then optimize. Hardly ideal. And the output doesn't exactly match the original PDF, with vector elements and text occasionally chopped around.