Check This Out!: https://www.kickstarter.com/projects/canopygames/magracer-a-fast-paced-3d-racing-game-course-using-godot?ref=6ffkee
I hate hidden problems like this!
You may have some unintentionally wrong transforms or vertices that are preventing your modifier from working. Double check the shapes you’re trying to modify, and try re-doing them using new nodes!
Hope you find this helpful.
My modular game assets course: https://www.canopy.games/p/godot-getaway-blender/?coupon_code=YOUTUBE_19-20