First let me explain the bait:
Scaled sardine: Silver body with darker back gray, blue, or green. Body is thicker/heaver than the thread thin, making for better casting distance. The eye is twice the size as the thread thins. Scales do not easily fall off, and with a good running live well they will last all day. Easy to keep on the hook, and stays alive for quite awhile.
Thread Thin: At first glance it looks like the scaled sardine, however when compared it's easy to tell the difference. They have the silver body, however it's much thinner. It's belly is deeply curved compared to the scaled sardine. Part of the dorsal fin has what looks like a long piece of thread towards the rear, hence the name. The scales shed easily, and despite a good running live well they don't last long especially in the warmer months. Easy to throw off the hook, and dies fairly quick.
Crazy thing about these two types of bait fish is they have several names like whitebait, greenies, greenbacks - I'm not sure why, I think it's just the neighborhood you come from. After netting a few times and comparing the two you will soon know the difference, and just give them any name you come up with.
Chumming:
The reason for chumming: Fish are lazy - one bait cast close to a lazy fish may not do much, however if there are a dozen or so baits flopping around the surface the fish are likely to get excited and start feeding. Chumming live baits can be against a mangrove shoreline, into a hole, or even on your favorite grass flat.
What to use: My preferred chumming tool is a hollow whiffle ball bat that has the top cut off at an angle. A wood or rubber stopper stuffed inside towards the handle will keep bait from getting stuck down inside the handle. I grab five or ten green backs - give them a little squeeze - put them in the bat and throw them into the fishing hole. If it happens the way you want, it won't be long before you see fish snapping the surface munching down on these baits.
As a norm you have to keep chumming every few minutes or so to keep the action going. It seems that you throw a few out, you get a fish - throw a few more out you get another fish. Just keep the chumming going and you'll keep the fish active. Give it a try - you'll be glad you did.