This usually happens because different slides are not sitting in exactly the same focal plane.

Common causes:

• Slides mounted backwards: the film image is on the emulsion side, so reversing a slide changes its position slightly in the gate.
• Different mount thicknesses: slides from different processors or mounts can place the film at slightly different distances.
• Heat from the projector lamp: slides can bow or “pop” as they warm up, shifting focus.
• Projector movement: vibration from the carousel or nearby foot traffic can also change focus slightly.

Ways to reduce it:

• Make sure all slides are oriented correctly.
• Remount slides into uniform, good-quality mounts so they all sit consistently.
• Reduce heat effects if possible, such as improving IR/heat filtration.
• Keep the projector stable and isolated from vibration.
• Increase effective depth of field: move the projector farther from the screen, or stop down/add an aperture to the projection lens if your setup allows.

If the problem happens only on certain slides, those mounts or heat-related slide popping are the most likely causes.