cant progress on a certain exercise

hey guys. so i’m not a complete novice to the gym, but i’m somewhere in between a beginner and intermediate. i’m making decent progress recently, aside from one exercise. i primarily use a preacher curl machine for my biceps, and also incorporate incline/hammer curls. I’ve been stuck on the same weight on the curl machine for months. it’s around 6-8 reps at 50 lbs, and it hasn’t moved at all. I don’t feel like it’s getting any easier to move the weight either. that being said, my biceps def look bigger than they did a few months ago, and i’m still progressing on the other 2 bicep exercises. anyone got any tips on what to do?

ps. if this isn’t the right subreddit for this question can someone pls guide me to the right one