1. Are you tracking calories? Doesn't have to be religiously but one should have a general idea of where they're at if the goal is mass gain and things have stalled. I'm not talking about weighing every gram of food you put in your mouth and meticulously logging your life on MyFitnessPal. As long as you're aware (within 100-200 calories) of what's going in, you should have an idea of what to adjust.
Most typical bodybuilding programs have way too many sets and reps and use the wrong exercises. However, if you lower the total volume, go heavier, and use compound movements as I’ve outlined above, there is nothing wrong with a body-part split for advanced lifters. In fact, it’s often less stressful to the joints than your average upper/lower split.
Note that this recommendation is for total weekly volume, which means it would need to be divided up based on how many times you’re training each muscle group per week. So, for example, someone training everything twice per week would do 30-70 reps for each bigger muscle group in each of those workouts, and 15-35 reps for each smaller muscle group in each of those workouts.