Check to see if the graphics card is seated properly.It may have moved whilst you swapped cases.
Make sure that you are using the graphics card slot and not the motherboard one.I know that sounds silly,but it has been done before.
Galaxy Note 8 vs iPhone X
The secrets of creating gory VFX
How to update iOS on iPhone or iPad
WhatsApp : comment lire vos messages sans que l’expéditeur le sache