Welcome to the forum. I grew Loofah for many years. I had a lot of fun with them and even ate a few. I mostly grew them for the sponge when dried.
I had very few problems with them. From what you are describing, it does sound like an insect problem or an insufficient pollination problem. I have no idea what insect pests are present in South Africa. From the little I've read about South Africa, I think you probably have more pests than we have in the states.
From history, when the Dutch and trekboers first settled South Africa and Rhodesia (Zimbabwe), they attempted to plant and grow the same crops or types of crops they grew in Europe. Because of the latitude closer to the equator and different insect pests, the crops simply would not grow. They nearly starved to death. That may be part of the problem you are having with your Loufah. I don't know where Loufah originated but I think it is more accustomed to climates in Taiwan and cooler climates of China.
I don't really think they are susceptible to squash vine borers. I say that because I would grow squash less than ten feet from the Loufah. The borers would attack the squash and not attack the Loufah at all.
I do know from my experience, the vines require huge amounts of water and nitrogen. I had to fertilize with lots of Ammonium Nitrate. I don't think it is possible to over fertilize them. My vines would grow up trees in excess of thirty feet and the fruit would exceed thirty six inches long.
I also seem to remember the Loufah is a climbing vine and does not perform as well as a vining crop on the ground.
I hope this information helps.
I simply enjoy gardening!