Taking it one step at a time you have to not get cought up in going in every different direction when trying to troubleshoot. So that means getting a check list and one by one check off what you have checked.
The SSD is in AHCI mode.
The boot order is with the SSD listed first.
In some motherboards there are severl parts to the boot order. One is for you to list the hard drive you want listed first. The next is for the cd/dvd drive that you want listed first. Then third is for the usb or floppy that you want listed first. Each one of these will show up in the boot sequence by taking the first one listed in each section. So if you want two hard drives to show up in the boot sequence then you have to select none in one of those sections.
The sata data cables connected to the drive and the motherboard.
The sata power cable connected to the drive.
If you take each one and check it off and you end up still in the same situation then there's a possability that the SSD may be defective of in need of a firmware upgrade.