It's probably something to do with the weird ID arithmetic you're doing in WorldGenBigCocoShroom. Block IDs aren't guaranteed to be sequential and can be different for every map.
That class is very hard to read because you're using old MCP mappings and haven't renamed any of the parameters or local variables. I suggest you update your MCP mappings and replace your current implementation (which seems to be a direct copy of the 1.8 WorldGenBigMushroom class) with a copy of the current WorldGenBigMushroom class (assuming you're not using 1.8, which you shouldn't be).
You should also rename any parameters that still have SRG names and any local variables that still have auto-generated names.
Please don't PM me to ask for help or to join your mod development team. Asking your question in a public thread preserves it for people who are having the same problem in the future. I'm not interested in developing mods with people.