My guess on what is happening is that all your PDF boxes (Crop, Bleed, Trim and Art) are the same size and are on the boundary of the press sheet. The crop marks in this case will be outside the press sheet and unviewable. If the trim box (the box most likely to be associated with trim marks) is smaller than the crop box and used as the FFCore imposition trim mark box (see below), the trim marks will appear on the press sheet.
One needs to choose a press sheet size. The size can be specified as one size, or one can use "Automatic" and specify potential sheet sizes that FFCore can do a best match to. If you add a trim mark, it can be edited for the pdf box used, length, style, etc. See below.
