Water stress is possible, but it looks like poor pollination to me. I've only grown luffa once, but I see it a lot in cucumbers, which are closely related.
When not enough pollen is transferred to the female flower to produce the number of seeds that the fruit would normally have, it will produce seeds starting at one end, and stop when it runs out of pollen. The rest of the fruit still forms, but with no seeds in it, the plant also doesn't invest energy in making the flesh to surround them, so you get a skinny fruit which is only fat where it has seeds.
The third one from the bottom is showing a little bit of the same effect, but only at the very top where it's much less severe.
