I suspect you're not as thrilled with the new lens because you are finding it harder to get sharp results. I believe you main problem is that you're handholding technique hasn't developed far enough for the longer focal length.
All things equal the longer focal length lens needs to be stablized better because the sensor is farther from the subject. Try to think of it this way. Say you have a laser pointer and you a shinning it on a surface 1 foot away and you are holding it as still as possible. The point will be very steady. Now point it at a surface 20 feet away. You will notice that the point will move around a lot more. This additional movement translate to a shot that is less sharp bacause of the additional distance. I can shoot my 90mm macro very close up handheld and get sharp shots but when I switch to my 150mm I switch to a tripod because my keeper rate drops dramatically handheld.
Don't get me wrong, I'm not saying you can's shoot yout 180mm macro handheld. It just takes a lot steadier hand to do it. You may be able to work on your technique and get sharp shots with the 180.