There can be multiple factors

1. Consistent muscle stress, not enough to cause pain, but enough to encourage growth. Ensure your doing just enough exercise to get to technical failure then stop.

2. Sleep

3. Stress

4. Adequate high quality protein, check the diaas score, but animal protein is best

5. Too much sugar in the diet, which messes with insulin

6. Mixing fat and carbohydrates, which causes inflammation

Basically the body needs to be in balance to build muscles.

Don't worry about hypertrophy. Use something like grip strength to track your progress, not muscle bulges.

Your lack of specificity indicates you have no clear goal and no defined plan. Your progress matches that: newbie gains that will hit a ceiling shortly. That's fine, newbie gains are awesome. While enjoying those, research so you can start taking your training from "well-intentioned newbie" to "beginner".

Getting stronger, having larger muscles, losing fat, and developing boxing-specific skills are all separate goals.

Pick a goal. Find a plan from a reputable source that is designed for that goal. Follow the plan. Make sure the plan isn't just working out, but diet and nutrition to support it too.

if you are looking for volume, research bodybuilding - its a specific approach. strength training is also different. what you are likely experiencing is endurance (muscle/cardio/etc) training in addition to the skill bumps.

fat wont hold back muscle gain, but it will hide it. good news is that as you more build muscle, you burn more calories. less mass means better cardio, which builds muscle endurance.

good luck with the boxing! its a fantastic martial art.

It take months, years... It is a long con game... Don't give up and keep working hard, nothing comes easy!

Do a summerjob digging dirt and lifting bags. And eat extra eaccordingly, and creatine shakes. This shit just works.

If you want to get bigger, you need to put your muscles under enough stress so their fibers break and rebuild stronger (you feel it as aching when they rebuild).

Lifting weights, chest press or anything with pushing or pulling at a high force can do this.

Something that has been effective for me is to do weight training being conscious of progressive overload. I do 2 upper and 2 lower days per week. I have a spreadsheet setup where i track each of the movements for each day, what weight i am at, what my target rep range is and number of sets, how many reps i achieved for each set this week, and how many last week, and some notes. Like just tips for form or a note to self to increase the weight. Using rep ranges instead of a single value for reps is important i think because as you move up in weight you will naturally be able to do fewer reps at the beginning. So, for example, you might do 3 sets of 6-10 reps of dumbbell bench press. When you can hit 3 sets of 10 reps you increase the weight and maybe you'll do 8, 8, 6 or something the next week.

Doing this you should be able to see your progress over time through increasing reps or increasing weights

Give these two articles a good read:

Link 1

Link 2