A missing joist label can occur if you copy and paste a joist that has already been group tagged.
The joist is set to suppress the individual label when grouped, and it maintains that setting even after you paste it.
One option is to right-click the joist and choose Text and Annotation > Reset Label Text.
Another option is to convert the joist to a beam, and then back to a joist, and the label will reappear.
Right-click the joist and choose Convert To Beam. Then right-click the beam and choose Convert To > Convert to Joist.