[Don Schaeffer]

How come so many people get nice sharp macros and I get these? I am using the Sigma 18-200 mm lens at f 5.6 or so.

[NT73]

Try single spot focussing, or manual.

Or find a big insect.

[Don Schaeffer]

lol

[Zig]

I wouldn't be hard on yourself Don: depth of field at macro distance is always going to be narrow, with f5.6 narrowing it down even further.
Also depth of field narrows the longer your lens is: thus, a 50mm macro will give you a wider depth of field(with more in focus) than a 90mm at the same aperture.
By the time one is doing a macro shot at 200mm, say, the depth of field at even f16(or smaller) will still be so narrow as to require razorlike precision.
Prime macro lenses are obviously ideal: they are optimised for close work and autofocus is generally very precise.
A "cover-all" lens like a 18-200mm is quite far from ideal for macros. Actually, f5.6 IS probably your sharpest aperture setting on that lens, though it's also not renowned for its macro capabilities. Given all this, I reckon you've done darn well to get these!
One sort of "workaround"(far from ideal, but fun) I do, is to try using my best prime lens at the closest focus I can get away with(macro or not) at the widest aperture. For me, this would be slapping on the 21mm, throwing the tripod away, sticking the lens right up the nose of a flower,etc; staying at f2.8 or 3.5 so as to avoid camera shake, and blatting away till I got one shot that worked. I might have about 3 keepers in 70 shots this way but that's fine.
I'd guess that if you put the Nikon lens on and shot at f1.8 or f2-ish, you'd be quite excited by the results.
I know it's a pain but depth of field is one of those yawny subjects it does help to do a lot of research about. Lots of fog to begin with, but the breakthroughs are worth all the hard work.
I'd even go beyond NT's succinct(but spot on) advice and say find a dead insect! Place it on a sheet of white paper in even light, dial in +2 stops of exposure compensation(as your camera will want to make the white paper 18% grey), either stand above it or use a tripod, experiment with distance and aperture settings with your sharpest lens(I'm assuming that is the Nikon 50mm, Don)
By way of example, I took this shot of a fly in exctly the same way as I described above. Not quite the pastoral idyll, if you like, but worked as an experiment.
I still reckon you've done pretty well with that lens, to be honest.

[Don Schaeffer]

Thanks. My "macro" is the Nikkor 50mm f1.8 with a Raynox Macco adapter attached. I couldn't get that close to the bee. I just used tIhe Sigma 18-200 and did the best could.