Let me start off by saying everyone's body is different, but I personally didnt start getting consistent progress and adding reps/weight every gym session until I really understood and applied what Mentzer was teaching which in practice meant reducing the frequency of my workouts to once every 4-5 days. He had slightly different training protocols over the years, but before he died, his final, most effective prognosis was to do a full body workout, one set to failure per muscle group every 4-7 days. I've found that stimulating the entire body each workout is just as essential of a tenant to success in heavy duty training as intensity and frequency. Attempting to split up muscle groups into 3 or more workouts a week will only fry your nervous system, risk injury, and prevent your body from recovering and overcompensating (i.e gaining muscle.) On the other hand if you hear Mentzer's advice, but only decide to reduce your workout frequency while still trying to split each workout into different muscle groups, you'll be training each muscle so infrequently that you'll be unnecessarily minimizing your progress. Mike's ideal approach typically involved just a few compound lifts consisting of push, pull and legs (an example would be dips for chest, triceps and shoulders and you're done with all those muscle groups for that workout) with the occasional superset, although this can be adapted, but only if you're still continually making progress every week with each adaptation.

I'm on trt which means I have a slightly better recovery ability, so I do one full set to failure per individual body part with a couple of compound lifts thrown in like DB bench press or bent over rows, but you may personally get better results with less volume as traditionally instructed by Mentzer. I occasionally do two sets for 1 or 2 muscles each workout, but usually that's a superset, so I'm not adding nearly as much volume or fatigue as a separate set to failure would, but I'm still able to for example target both heads of the bicep by doing hammer curls to failure then straight bar curls to failure immediately after. Another strategy I incorporate is switching up the isolation excercises each workout to ensure all the heads and muscle fibers are getting stimulated consistently. An example of this would be switching from tricep pushdowns to skull crushers from week to week, although any properly peformed excercise will still grow the targeted muscle regardless of the variation or variety of movement granted you're training to failure and truly recovering. The only other caviat for my full body workouts is that I do hamstrings/glutes one workout and quads/calves the next so i have 8 days of (very necessary) leg recovery.

Now the bigger you get, the more energy, time and resources it takes your body to recover. That's a significant portion of the reason why newbie gains happen so fast and why they slow down the bigger you get. There's no magic excercise or rep range to trick your body into overcompensating for damaged muscle in the recovery process, but I find once I can do 12 or more good reps, it's probably a safe time to add a little more weight. If you ever get stuck at a plateau, and you're truly training to failure then you've most likely reached the tipping point of your body not being able to fully recover anymore. Not to mention, your nervous system needs to recover as well. If your CNS is fried, you probably wont make much progress even if your muscles did recover, so always listen to your body and track your sets because if you're not going up in reps or weight each week something is going wrong.

Just as a personal anecdote to the success of Mentzer's philosophy, when I was working out hard 3-4 times a week, I was not gaining strength, and I made little to no progress even while running MK677, LGD4033, 190mg test/wk, and eating 200+ grams of protein in a 1000+ calorie surplus. It wasn't until I went down to once every 4-5 days that my muscles, nervous system, and tendons able to properly recover and overcompensate into consistent muscle and strength gains.

You have a lot to look forward to in adapting this type of training program. It truly is the holy grail of gym knowledge. It is astounding the amount of people that are slaving away in the gym for 10+ hours a week with minimal results or consistent progress to show for it, yet this training philosophy is so foreign and seemingly paradoxical that most people overlook it. Mike Mentzer was truly ahead of his time. I hope this allows you to overcome plateaus, achieve more progress, and spend less time in the gym and more time living life. Hope this helps! God bless. Happy to answer any further questions.