CLion/CMake can find SDL in one project, but not another - c++

Question

I have two C++ projects in SDL, both using SDL. For whatever reason, one of these projects can find SDL just fine. For the other, I can write code as if SDL was found- with autocompletion and everything- but when it comes to building the project, I am informed that fatal error: 'SDL2/SDL.h' file not found.
The CMakeLists.txt files of both projects are basically identical.
CMakeLists.txt for the project that doesn't work:
cmake_minimum_required(VERSION 3.12)
project(Snake)
set(CMAKE_CXX_STANDARD 11)
include_directories(.)
find_package(SDL2 REQUIRED)
add_executable(${PROJECT_NAME}
main.cpp
SnakeBlock.cpp
SnakeBlock.h
SnakeGame.cpp
SnakeGame.h
SnakeBoard.cpp
SnakeBoard.h)
target_link_libraries(${PROJECT_NAME} ${SDL2_LIBRARY})
And CMakeLists.txt for the project that does work:
cmake_minimum_required(VERSION 3.12)
project(Tetris)
set(CMAKE_CXX_STANDARD 11)
include_directories(.)
find_package(SDL2 REQUIRED)
add_executable(${PROJECT_NAME}
Game.cpp
Game.hpp
main.cpp
Playfield.cpp
Playfield.hpp
Tetromino.cpp
Tetromino.hpp)
target_link_libraries(${PROJECT_NAME} ${SDL2_LIBRARY})
And for both projects, SDL2 appears under the "External Libraries" section on the right sidebar, under Header Search Paths.
What gives?