@greg67 Two things:
What's failing? Grip? Forearms/arms? Shoulders? Back?
Sounds to me like two things: either too low volume and effort, or too high volume and intensity.
Get back to two sets twice a week, get your reps way lower at first. Build reps back up slowly. Once you reach a plateau, increase the number of sets. Let's say you hit the same plateau and can only do 8 reps on your first set. Add a third set while staying at 8 on the first. Then a fourth, then a fifth, then add an exercise like horizontal rows for a couple sets and build back from that. This is a good 2 months progression. Hopefully you break that plateau from there, but if you don't, you take a week or two off, and you start again, two sets, low reps, etc... Out of 6 months of this, you should have an idea of how much volume your back can take and recover in 2 days. You can then focus on repeating the same loading/deloading cycles with a volume that feels more comfortable (let's say you like 2 to 3 sets and 2 exercises, you do that)
Obviously, prioritisation should be reflecting what your objective is. Pullups right at the start of your workout. Also, you should put slightly less effort on the rest, at least when pullups are in the heavy part of the cycle.
Progress is made when you rest (sleep, eat) after a strong stimulus. Sometimes too much effort also kills gains. You need to learn when to try hard and when to enjoy your training without pushing your limits.
Last note, isometrics and negatives do help. They are just super painful and don't really help to grow muscle so people ignore them, but they are key to increasing relative strength.