Errors in rasterization during "Export Map to AI"

Hi

I have recently noticed problems with rasterization when exporting a map to an AI file. I have a number of vector & rasters included in the export and for some reason line & polygon files such as coasts & admin boundaries get merged into the raster (srtm) during export. When I open the AI in Illustrator, the image layer contains the merged raster which means I cant edit each individual layer. This error doesnt happen all the time and not all vectors are affected: points and all labels are separate. It even merges the vectors when no raster is included. I have picture symbol set to vectorize layers with bitmap markers/fills

Any ideas?

Thanks
Lauren