Particles have level of detail so they appear differently with different graphics settings. Consider the properties of your particles and your Lighting which will affect how they’re rendered and try adjusting them to see if you can get a good fit instead.
There is also a maximum number of particles that can be rendered on screen at one time, so if you have a particle intensive game then you may be hitting the draw limit. Not enough information here to understand your circumstances so I can only give a broad answer like this.