JPG and RAW images not working together on Import/View

Are you actually shooting raw+jpeg in camera or are you using the camera to create edited jpegs? In the latter case, Lightroom Classic import might not detect they are the same file as they will have a different creation date.

If they were shot raw+jpeg try resetting preferences indeed. For already imported images, use the auto-stack function with a zero time difference to automatically stack the raws+jpegs together. 

Edit: forgot about the synchronize folder trick that Johan documents above. That will work too but will lose access to the jpegs as they will only be used as sidecars from then on.

Just a thought, you stated " in some cases, not seeing that the JPGs and RAWs are the same files - and is importing them as separate files".

I you have selected  to save raw and  jpeg  images in your camera then there are two images one raw and one jpeg.

 You also indicate that you selected an option in LrC prefrences to treat raw and jpeg, I am not aware of that option. There is an option that you can select "Treat jpeg files next to raw files as separate photos ".

Try resetting the preferences: https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/

If that helped for new imports, then using 'Synchronize Folder' should marry these couples back as sidecars.