TweetI'd like to know more about your diet. For that matter, what does your training look like? Unless you weigh about 50 lbs, 100g of protein is not a high amount of protein. 2x bodyweight is more like it.
There's a saying "....you don't grow in the gym". Growth happens during rest time, and only if your body has the nutrients that it needs to support growth. That means, there's no real concept of a "grow-day". You need to eat properly everyday to support muscle growth.
I have a theory: I'll bet that you're arms are lagging, because you're overtraining them. Seriously, if you train back right, there's no need for a "bicep day". You could add a few sets at the end, but not a whole workout. Same goes for chest - if your chest workout is dialed in, there's no need for "tricep day".
Most people get fat on insulin, because they (understandably) go overboard on carbs.
Once a week, IMO is not going to do anything for you, honestly. Use diet and training to bring up lagging parts.
Understand, that I am NOT speaking from personal experience - just what I've learned over the years. You may want to wait and hear from bros who have experience with slin.