Bicep weight progression

I'm having difficulty with my progress with biceps. They've always been my weakest point, but lately I'm stalling every other week on these lifts.

I'm doing ICF, and it's a very slow crawl. I'm supposed to be progressing on standing straight bar curl 3x8 every other workout, but I'm actually seeing a 5 lb raise every 4 to 5 workouts, and the first time I increase weight I am pretty much guaranteed to fail early on into not the third set. Next few workouts get me closer to finishing, and when I finally do finish the set and add weight.. I'm right back to failing.

Anyone got advice on how to make more solid gains here?