well,sir, I squandered much time with the weights on and off for a few years...because I was using a muscle-group split mentality..frankly, I realize in hindsight how retarded that was/is. but, how could I know when I was just following what I saw others doing and wutnot?
rest doesn't mean a good night's sleep, either...it might mean taking a week off if you've been at it for 5+ weeks straight pretty intensely...
but, if you include compound exercises such as the squat/deadlift/presses/chin-ups/rows as staples in your routine and limit isolation exercises to a very minimal role (20% or less maybe?) in each and every work-out, AND foucus on progression you should make steady gains.
also...
any split-based routine that makes any sense to me these days is either a push/pull or upper/lower...and, of course, without splitting a full-body routine is always solid. years down the road when the mass has been built, it might make sense for a little more concentration on isolation/muscle group (routine)exercises to chisel out a certain look.
anyway, gains without being on the 'juice' for people with average genetics or worse are not automatic...people fumble around with working out 5+days a week and doing muscle-split routines or don't focus on progression and then burn-out...mostly, though, if some key fundamentals are followed, gains can and should be steadily realized by about everyone.