If you're not seeing results working a body part once per week, I recommend trying to work that body part more often. Soreness doesn't really enter into the question unless it is debilitating.
I have not personally found that exercise variety makes a big difference for most adult men who are relative beginners, like you and me and everyone else bench pressing less than 1.5x their bodyweight. Exercise selection matters, because you want to pick an exercise that is effective for your needs and goals. But once you identify that exercise, I see no reason to do a bunch of similar stuff. For instance, as a beginner my arms grew just by doing overhead press, pull-ups, and dips. (And on some programs not even all three.)
One key reason to focus on one exercise rather than several is that it lets you spend more energy in that one exercise. Four sets of 12-15 reps for four exercises means you need to spread your ATP across 192 to 240 total reps. With three sets of 12-20 reps (my favorite for dips) you can focus all your effort into making those 36 to 60 as heavy and intense as possible.
If your triceps are not getting bigger and stronger with consistent once-a-week training, try working them every workout. One exercise should be plenty. One challenging set might be enough, but I prefer two to five, usually three. Three hard sets of a heavily-tricep-dependent compound movement, done three or four times a week, a little more challenging each time (i.e. aiming for a few more pounds or reps per set each workout) should either get you bigger and stronger or tell you something more fundamental is wrong. Once a week is just not very much!
