Converting to Bitmap issue

Hallo

I do large format printing and having colour issues as soon as I convert images to Bitmap in Corel.

Attached is a image imported to Corel and exported to PDF for print and the colour is perfect. I then took the same image and converted to a bitmap (As we do when adding transparencies etc) and exported in the same way to a PDF but all the grey colours seems to be black.

Please can anyone help?

Thank you