HELP ME REVIVE MY AVOCADO PLANT

Hello all!

I would greatly appreciate any advice on my plants!

My avocado plant needs rescuing, what can I do? The tip is turning brown. I've seen this before because I have gotten advice to cut the stem for it to grow fuller, however, when ever I did that it would kill the plant in this same way.

The plant was in a small pot and needed repotting as the roots outgrew the first small pot, and it dropped its little leaves.  So I repotted the plant in a slightly larger terracota pot.

I do not belive that I over water or under water the plant, and I don't know why it is sick.  You can also see my other avocado plants, they are slightly healthier, and I give them all the same care.

Please view the images below and I would appreciate any advise.

Thank you!