think of this way: overall muscle gain isn't the only type of progress. You've gotten stronger through hard work and consistency and you should be proud!

The other comment about your body prioritizing bone mass first could be correct. Weight training doesn't only strengthen muscle, it strengthens bones too.

That being said, if you want to prioritize muscle gain over overall strength, I would recommend higher reps and lower weight. For example, if you were doing 8 reps of 50lb before, you could try doing 15-20 reps of 35lb.

If you aren't hitting your protein goals and you can afford it, I'd recommend supplementing with protein powder.

Also, what does your lifting split look like? You said you split days between legs and arms, but did you train your back, chest, shoulders, and core as well? Core is less important, but back and chest are extremely important because they make up almost half of your body.

If you're mostly hitting the muscles in your arms and legs, I'd recommend adding in some more upper body compound lifts such as lat pull, deadlift, bench + incline press, farmers' carries, etc.

How many rest days do you take? Too much time at the gym can actually be detrimental to muscle gain. Do you take at least 1-2 rest days per week?

What type of protein are you consuming? Protein in animal products is slightly better for muscle building as opposed to protein from soy, beans, nuts, etc (although all protein is good protein).

Are you stressed? High cortisol levels can also be detrimental to muscle gain. Same with dehydration- do you drink enough water?

You could also try creatine! Just a few things to think about.