Question

Why do clouds, once seen at a particular point, may not be there after some time?

Answer 1

It is so because, the clouds actually floats in the air.They  are too light.
So, we cannot see clouds at a particular point.

Answer 2

Clouds, once seen at a particular point, may not be there after some time because they are always in motion due to prevailing winds of our planet.  

Explanation:

Prevailing winds are wind caused by earth’s magnetic field and temperature difference of our atmosphere, there are 3 prevailing winds which flows nonstop, we might not fell them on the surface but due to lower air density the winds are much more stronger higher-up where they blow nonstop, Hence, due to constant motion of air or wind in the upper atmosphere clouds move constantly.