Crash before the main menu
Check the crash report for version or dependency errors. Remove duplicate files and confirm every mod belongs to the same loader ecosystem.
Game loads but features seem missing
Make sure you launched the intended profile, the mod is actually present in the loaded mod list, and relevant features are not disabled in configuration.
Problem appears only in one world
Back up that world, compare configs and test the same setup in a fresh world. This helps separate a world-specific issue from a general installation problem.