Two issues. The first. It is not possible to increase reps, and/or weight, every workout, or you would be benching a thousand punds for high digit sets....The normal Powerlifting protocal, produces by far, the greatest stregnth gains, and muscle hardness...3-5 reps...few sets...
Most of stregnth is learning to generate muscle tension, Only bodybuilders need to make it grow beyond the bounds that making it super strong, will produce.....I.E. At 5'8" 200 lbs 15% BF, My arms are still over 16 inches, and get over 17, when buffed out...And I have yet to break 400lbs in the deadlift.
Occaisionally, everyone needs a rest...That's Periodization. that is what effects optimal stregnth/growth or whatever you may be seeking....
Two. Yes, once upon a time Mentzer was really great. But that was not with out the aid of pharmacology either...something to consider.....I own a copy of Heavy Duty, and there is some useful information in it. But likewise, Power, The Poliquin Principles, Ian Kings book...(I forget the title offhand) as well as Some of the Stuff Pavel is spouting...(Especially the Kettlebell thing...) most of his other stuff seems to be a re-hash of info that can be found in other places...But that's OK too.
of course not. that would be absurd, and goes without saying. that gains should be continous within the potential of genetics when adequate rest time is given is all the point entails, and is valid. it means, in order to gain reps and strength, recovery must be sufficient to lay down new muscle tissue. most systems have folks overtraining.
if you havent increased in reps or strength since your last workout of the muscle group, only a few things will account for it: incomplete recovery, which involves a ton of variables, inadequate stimulation of the growth response, that is, the need to stimulate other types of muscle fiber, or genetic limit.