Has a chance to spawn without the torpor stat, making the particular individual Rex unable to be knocked out and therefore untamable. Check to see if they have torpor or not using your telescope before opening fire. Probably a glitch.
If you see an alpha rex please be careful, they are unbelievably scarier than a normal rex keeping up with a spinosaur and not getting phased by anything we threw at it. Rip Zeus, level 113, the rex was 25.