The most likely cause is pre-flash. Many point-and-shoot cameras fire one or more small flashes before the actual exposure for TTL metering, red-eye reduction, autofocus assist, or wireless control. A basic optical slave sees that early flash and triggers immediately, so by the time the real exposure happens, the external flash has already fired and contributes little or nothing.

That makes the image look flat or weak because you’re mostly seeing the built-in flash, not the off-camera unit.

Ways to fix it:

• Disable pre-flash features if your camera allows it (such as red-eye reduction or TTL flash behavior).
• Use an optical slave that can ignore pre-flashes or “learn” the flash pattern.
• If your camera always pre-flashes and offers no manual flash control, this setup may be unreliable with a simple slave.

So the issue is usually not that off-camera flash is inherently softer here—it’s that the slave is likely firing at the wrong time.