Defog Doesn't Work Properly

[Former user]

Defog lowers the opponents' pokemon's evasiveness, but that's all it does for me. I am challenging the bug/poison type gym and I try to use defog to get rid of the stealth rocks and sticky web the shuckle puts up, but it fails to remove them. I'm not sure if I'm just being stupid here, if there's some ability, field effect, or move that stops me from doing it, or if defog is broken for me. Any help would be much appreciated!