@enkonyito

The difference between the 1st and 3rd image is that caustics are not visible in daylight with Sunflow rendering engine. I don't know why.

According to the Sunflow wiki

It's also important to note that at this time IBL and Sunsky do not emit photons, therefore, no caustics are viewable when using these lights.

I haven't dug through the code enough, but it appears time of day (and, presumably, Sunsky vs IBL) can affect renders even when the scene receives no illumination from the sun. In tests of a rendering issue I finally discovered was caused by mapping different materials to the front and back surfaces of a SketchUp-originated model, I found the same scene rendered differently depending on time of day -- even when the scene was set within an opaque box.

Even though neither sky light will generate caustics, you might want to try rendering your scene using a textured sky (or without a textured sky, if one was present for your first render), just to see if one of the two sky models was somehow blocking Sunflow from rendering caustics generated by other sources. Knowing that might possibly help locate and correct the problem within Sunflow.

maz