Lightroom masks not working on 85mm shot jewelry images

Hi there - I'm shooting small pendants with a Sony A7 IV and and 85mm 1.8 lens on a white background (under and back lighting with soft box to the side and a v-flat light cone to diffuse).  Shooting at around F5.6 and ISO 100 in RAW.  Problem I'm having is that when I import the photos into Lightroom to edit, I can't mask off the background (to get a true all white background).  I'm wondering if it's because the image of the pendant is so small that even thought I crop it down when I try to apply a mask it doesn't detect the pendant.  I can't be the first to shoot a small product with an 85mm lens.  How can I get Lightroom to mask off the background of an image that is small relative to the overall picture??