My experience with a cast net is that they work well only when you have fish concentrated. Either by knowing where a school is or using it in a confined area where there is already a concentration of fish.
Fish are pretty thick in this 1/20th acre forage pond. Still, I have to feed the GS to get them grouped up to get very many per throw. Even then at times I will get a lot in one throw then maybe just a few in the next.
When casting for RES fingerlings, I found that waiting till almost dark (last fall) the fish would be coming up into shallow water to feed and I could get acceptable numbers per throw. In mid day casting to the same area I would get very few or none. I also found that if the water was cold and the fish sluggish it helped. In hot weather when the fish are very active, I think a lot of them can flee beneath the weights before they hit the bottom. Fish are quick. If a person is really into cast nets, you want to use the biggest mesh that will work for the size of fish you are targeting. The reason is, for a given amount of lead weight, a coarser mesh will sink faster than a fine mesh. Too fine of mesh with a too small diameter net will mean the net sinks too slowly and fish will swim out from under the weights before they hit the bottom.
So just like fishing with a hook, bait and line, knowing where the fish are, casting appropriately, and having the right size/mesh/weight ratio are all keys to success, in my opinion. I am not always successful. But I have found a few specific instances where it works pretty good.
Stay away from artificial structure. It plays havoc on a cast net. And my net repair crew does not do outside work and is high maintenance .
And my net repair crew does not do outside work and is high maintenance 
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