Stubborn biceps

It's been 6 months now I've been doing this bicep routine twice per week:

3x to failure single arm db preacher curls

3x to failure single arm db incline curls

3x to failure chinups

I've been able to progressively overload somewhat with weight and then plateau with weight and then slowly been able to add a few extra reps over time.

so I've felt my biceps have become stronger but they just never grow in size. still noticed essentially zero hypertrophy. My triceps have grown significantly but my biceps just do not wanna grow whatsoever. Any suggestions? Does that bicep workout I listed seem legit or should I modify it? Drop weight? increase weight and less reps? etc