Progress. It's the name of the game. In everything and anything you do, you are continually working to improve your output. At your job, you seek increased means of productivity, resulting in company market share growth and a fat bonus and promotion for you. In your love life, you're seeking greater connection and selflessness, which leads to a happier relationship with your significant others. For your studies, you are always looking for better ways to learn the subject matter so that you can excel on the objectives of the course rubric, thus earning an "A" and placing yourself in the best possible position for career success.
When you're training in the gym, things are no different. You want to be the best possible "you" that you're capable of reaching. You want to meet your body's physical potential for muscular strength, size and appearance. And you want to do it, like, yesterday! You're not content to take your sweet time and let the gains come to you. Rather, you are keen on pushing yourself each workout to train longer and harder so that you can be bigger, leaner, and stronger. And the #1 way to improve as fast as possible is to use MORE WEIGHT with each workout.
Progressive resistance is the name of the game when it comes to muscle-building success. This is no secret. You've known since your first day in the weight room that the really big guys also happen to be the really strong guys. They all started just like you, but through consistent and small incremental increases in the amount of weight they were lifting, they were able to grow bigger and stronger. And you strive to do the same. When should you increase the amount of weight you are using? The most simple answer is "all of the time". You are a bodybuilder and your body - today - looks exactly as it needs to look in order to complete the workload you have subjected it to thus far. If you had trained heavier and with more intensity, then you would be bigger today. And that is indeed the goal going forward!
Be sure that you're making your incremental increases with nearly every workout - in very small increments. Your gym has small plates of the 1/2, 1 pound, and 2 1/2 pound varieties. These tiny and sometimes embarrassing weights need to become your best friend. Use them with every workout, forcing yourself to lift 1, 2, 3, 4, or 5 more pounds than you did the week before, for the same number of repetitions. If your gym doesn't have the small weights, then you should buy them online for a few bucks and bring them to the gym on your own! Your body isn't going to be able to magically move up 90 pounds on your bench in a single week. However, it will move up 1 to 2 pounds each week on the bar, which can equate to that same 90 pounds over the course of a year. Most people in your gym haven't increased their bench press strength by 90 pounds in the course of the past year, but they'd be thrilled with that outcome. Using the very small weights, tracking your lifts, and slowly adding to the bar each week is exactly how you attain these kinds of gains.
Your "8 rep max" is essentially your money maker. It is the target repetition range you should desire with most major lifts. Use it as your guideline for when to add weight and when not to add weight. If you're only completing 5 repetitions with a given weight, and your goal is to build new muscle mass, then you are likely using too much weight. You should cut back the weight and focus upon reaching 6-8 repetitions, ideally eight, with a bit less weight. As soon as you can complete 8 perfect repetitions, then it is time to move up the weight a bit. At the same time, if you can easily complete 8 repetitions, then you probably already know it's time to begin increasing the weight in very small increments.
Increases in workload don't just pertain to the amount of weight being moved. If you move 400 pounds for 8 repetitions one week, and then you move 400 pounds for 9 repetitions the following week, then you have most certainly increased your workload. Sure, the weight is the same, but you're moving it for a greater number of repetitions, thus increasing the workload upon your muscle fibers. One cannot just increase the repetitions indefinitely
Usually, a good rule of thumb is that once you are able to move the weight for 11 or 12 repetitions, it's definitely time to bump up the amount of weight you are using. If your target repetition range is 6-8 reps and you are adding very small amounts to the bar with each workout, then you're likely not going to move into that 11 to 12 repetition range. You'll have increased the workload adequately previously, which is always the goal!
Nothing lasts forever. You will hit a wall. There will be days, despite perfect supplementation, diet, recovery, and incremental lifting, when you still are unable to move an added 1 pound on the bar that week. That is a completely normal part of the process. The closer you move to that imaginary muscle building ceiling, the more slowly the gains will come. Keep training, mix up your exercises or training days to shock the body, and stick it out until you start seeing those small incremental gains again. Small gains add up over time!