Mac mini mid 2010 does not see new memory card installed

All Mac's including the Mac Mini's are very picky.

see > Upgrade or install memory in your Mac mini - Apple Support

What RAM modules are you trying to use in the 2010 Mac Mini?

see > https://eshop.macsales.com/shop/memory/Apple_Mac_mini/DDR3

Just one 4GB module or 2 2GB modules?

Did you buy the module(s) directly from OWC or somewhere else?

1. Double check the spec's to make sure that it is correct for your 2010 Mac Mini.
2. Make sure that you are fully seating the module(s) in the slot(s) before lowering them down into the clips.