I think you're looking at the wrong field. Block force is 15, but parry block armor, which I believe is what is used to determine if you successfully parry or not is a bit better than a silver shield, and you (I?) can definitely parry fulings and deathsquitos with a silver shield.

Maybe you don't have enough armor and/or health? I don't know for sure if that affects parry or just stagger, but that's all I can think of that could be the reason you can't parry with it.