"Development" and "Maintenance" Status
You can't significantly improve every muscle group simultaneously. This is why bodybuilders often use specialization cycles for stubborn muscle groups. But remember, specializing in one area necessitates putting other things on "back burner" status.
One simple way to accomplish this is to always assign each muscle to either "development" or "maintenance" status for any given training period. In practice, this mostly means distributing less total work to maintenance muscles and more relative work to those muscles you're aggressively trying to improve.
Although determining your MEV (minimum effective volume) and MAV (maximal adaptive volume) for each muscle group will take a fair bit of trial and error, one simple starting point would be to perform 9 sets a week for maintenance muscles and 15 sets per week for development muscles.
Run those numbers for 6 weeks and then re-assign each muscle to the opposite grouping for the following 6 weeks.
The Volume Method
Furthermore, there's a simple and reliable way to determine if the volumes you assign (in number of sets per muscle/week) are optimal:
- For maintenance muscles/exercises: If you can still hit the same number for the entire 6 weeks (meaning, your strength levels haven't receded), you probably haven't lost any muscle. And if you find yourself actually getting stronger over the 6-week maintenance phase, you can slightly reduce your weekly workload.
- For development muscles/exercises: You should be getting gradually stronger over the 6-week development phase. If, for example, you benched 205 for 3x12 on week 1, you should be able to reach perhaps 220-225 for the same sets and reps on week 6. If you can't, you're either working too hard or not hard enough.