If I understand you correctly, Projectile->TrailParticles is a UParticleSystemComponent pointer. Does Activate() work:
Projectile->TrailParticles->Activate();
My setup is a little different in that I have Auto-Activate off and then I enable it when needed. In your case though you’re activating it (and it works right?), then deactivating it, and trying to reactivate it again (which isn’t working).