If you own a Dell Latitude 6420, you know that when you upgrade the HDD for a SSD the hard drive is thicker than the SSD and won't install properly.
These 2 - Steps will enable you to install the new SSD properly.
1 - Once the SSD is in the caddy, push up as you slide it in the bay. Then the SSD will be firmly placed in the port inside.
Important: You must "push up and slide" because the SSD is thinner than the default hard drive, so its not as snug a fit.
2 - Go into your BIOS and go to "boot sequence" select UEFI: INT13(RAID,0x80) and select apply, then exit.
Once your computer has rebooted everything should work.
This is a solution that worked for me there could be other reasons that the SSD does not get detected. See the rest of the forums here for other solutions. I hope this helps.