Hi, DaveLiming!
Thanks for reaching out to Microsoft Community. We are sorry to learn you have issues with the microSD slot on your Surface Pro 6.
As what you described, it seems this is a hardware issue of the slot itself. If you have tried inserting several cards that are working on another device, there could be a pin that doesn't get in contact with the microSD card to keep it in place. If that is the case, a replacement is provided since Surface devices are not repaired. You may check the page on How to get service for Surface. As an alternative, you may use a USB drive with a microSD slot though.
Please visit this page https://support.microsoft.com/contactus/ to contact Microsoft Support. If you are outside the US, you can check this https://support.microsoft.com/help/4051701/ page that has the Global Customer service phone numbers for Microsoft.
If this answers your question, please let our Community know by marking this post as an answer. Otherwise, don't hesitate to respond to this thread if there’s anything else that needs to be addressed.
Kind regards,
Janette_Z