Are you eating enough surplus calories? That's the main thing that draws growth. If you eat too little, you can be having even double the protein you need: your body won't grow. Do you struggle to eat enough food?
From the spiritual point of view, you can inhale red energy for a comfortable amount of breaths, preferably before and after training. You can inhale it either directly in the muscles you used used, or generally into your whole body. Program it to ensure your target level of muscle growth (i.e. Muscle mass = 80% of your body), and remember to add that it happens in beneficial ways for you. Remember: it's important to state that it has happened and it keeps being that way. Visualise your body the way you want it.
You can also use the Uruz rune. It's perfect for beneficial body growth and athletic performance. 10 repetitions should be enough. If you can't do 10 yet, you can do less such as 2-5 or 8. Slow, but without discomfort, and fully focused on your goal. Affirmation and visualisation considerations from above apply here.
If you do either of them or both for a couple of months and it doesn't work, consider doing the affirmations at 2 extra points in the day, with no need to add red energy or runes: as soon as you wake up before you even move from your bed, and just before you fall asleep at night.
Much of your muscle growth happens when you're sleeping, as that's when your body consolidates any memory and growth. If you are going to bed too late and/or sleeping too little and have other factors debilitating your sleep, your muscle growth can be compromised. Just like anything else.
Sleep is the base of the pyramid in health, then comes diet, then come other lifestyle habit. To make the best changes, both sleep and diet need to be on point. If they're not and you refuse to implement changes (or can't), no matter how many workings you do, the results will be suboptimal at best.
Other things you can do is fill your food with energy and programme it for your goal. Even vibrate runes into it if you want to. Before you eat. After you eat, you can also program your digestion and metabolism the same way.
Make sure you're actually focusing on what you're doing instead of rushing through it. Rushing brings minimal results if any.
Hope this helps achieve your goals.