From what I've experienced and read, threatened doesn't always work, making sneak attack feel glitchy.
I'm not sure if it's currently true, but in earlier patches the "threatened" radius (>5 ft) was larger than the "adjacent" radius (<=5 ft). A character who is threatening wouldn't necessarily get an opportunity attack against said enemy if the enemy moved away. I presume that the same holds for sneak attack: a "threatening" ally isn't necessarily close enough to allow sneak attack to function.
Can anyone confirm that the threatened radius is still >5 feet in the current patch?
Threatened has definitely been changed (for the better), but I can't tell if it's still greater than 5 feet. It seems like there are more conditions to be threatened than just proximity now.
Right is a totally different thing. But regarding the no "you don't need advantage if another enemy of the target is within 5 feet", I tried really hard to do it to no avail
Have you tried having an ally attack the enemy before the rogue? Sneak attack is definitely buggy right now, and I usually make sure my fighter attacks before the rogue so I can get it.