ICs can be weird about how sensitive they actually are. I'm using 4 different brands of tests right now. FRER gave me a good positive at 10dpo (15miu), one IC dip strip gave me a vvvfl (10miu), one barely showed anything (25miu), and one IC midstream gave me a vvvfl (25miu). At 13dpo, my first brand of IC dip strips showed some progression but not a ton, the other still barely registered a positive at all, and the IC midstream gave me a nice strong BFP. So the ICs that gave me my first and strongest positive of the three IC brands I'm using isn't great for showing progression (even though it's supposed to be the most sensitive of all my tests), one is next to useless and barely shows anything at all ever, and the third wasn't good for an early positive but has been great for progression.
Go figure!
Anyway, what my rambling here is getting at is to not feel too discouraged if your ICs aren't picking up on much. They're cheap for a reason!