And a number of other reasons discussed in the forum.
Usual response:
Provide more background, code, screenshot, … to save us from needing to guess around 
Just a few
Particle Dev won't build, particle-cli will
Particle Dev: "build didn't produce binary"
Particle Dev with multiple projects - what?
Particle dev will not compile
Particle Dev objbase.h no such file Cannot compile
Mostly the same reasons 