My guess is that either a) your workouts have become stagnant in that you repeatedly do the same type of workouts (I'm not even a big believer in "muscle confusion" or "tricking the body" but there IS something to be said for it) or you have hit a plateau and aren't continually increasing your weights. You need to continue to add more resistance to your lifts.
I've learned a lot from my schooling, but probably more just from reading research articles and websites like bodybuilding.com. They have so much free information there (almost like fangraphs!). If you're interested in learning for yourself, I would suggest surfing it for a bit and seeing what you can find. You can find recipes, workouts (goal specific), etc.
Tldr; continually increase your weights in the gym, track your macros (fats, proteins and carbs), keep track of your lifts, and get a partner to hold you accountable, along with being a spotter so you can work til fatigue.