"Poor background removal" - if this is a refusal reason that you commonly receive when your images are reviewed, especially on subjects that have a plain (often white) background, here are a few tips that I hope you will find helpful.
If you intend to "cut out" your subject matter and place it on a white background, you need to make sure the background is actually white. And the edges of your subject need to look realistic - often they have the appearance of being cut out with scissors because the edges are too sharp - this is where your edge selection needs to be feathered by one or two pixels to soften the edge a little and give it a more natural appearance.
Often there are remnants of a background which are only noticeable by very close inspection. They might be around your subject, or they may appear in the corners of your image, or perhaps even in random areas.
It is always wise to review your own images before submitting them, and making sure that if you have a plain background, there is no contamination from leftover pixels. When you look at your images, check them at full size - 100% resolution, because that is how they will be inspected by a reviewer. If you view them at a reduced size, you won't notice problem areas.
There are a number of ways you can select your subject to remove it from the background. Photoshop's Pen Tool is extremely useful for objects that have naturally hard edges. It's not at all appropriate for hair on humans though, because hair is wispy and extremely time consuming to select. Or you can use the Quick Selection or Magic Wand tool - but the mask created will almost always need further refinement with these options.
Photoshop 5 and 6 do a pretty good job with selections and isolating subjects - Topaz Remask is also excellent. Each of them allow you to make a basic selection and then refine the mask to extract as much detail as possible. Remask is especially good with hair and fur.
A really good way to check your backgrounds after you have isolated, cut and pasted your subject into a plain background, (or if you've shot on white already and just need to clean up the background) is to check the levels or curves once the image is flattened. A quick and easy way to do this is to pull the levels or curves right down to darken the image - I actually use a free program called FastStone, and just hit Ctrl and M on the keyboard. That will bring up a Curves box and I can quickly drag the curve down and to the right to darken the image...and voila, then I can easily see whether my background is completely clean, and whether my isolation technique has worked well, or whether there are leftover pixels that need to be cleaned up.
Below is an example of a subject shot against a pale background...and then a copy of the same shot with the Curves adjustment. You can see a halo of grey leftover pixels around the hairline. The third image is an example of the "scissored" edge - it's hard and looks totally unnatural.
Bear in mind, the Curves adjustment is just to check your image - you don't save the image with the adjustment in place. If you use the Curves or Levels in Photoshop, use them as a separate layer which you can delete when you are satisfied with your image. If you use FastStone to check, you can just click on Cancel, and the effect won't be applied to your image.
Another common refusal for isolated subjects is the lighting - an object on white should have soft, even lighting - it should have the appearance of being shot in a studio. If the lighting is too dark or harsh, it detracts from your subject and will most likely result in your image being refused.
Lighting is worth remembering for subjects you shoot outdoors and then want to put on a white background. If your subject was shot on a sunny day and the shadows are deep and the light too harsh and contrasty, there is little likelihood it would be acceptable for stock. It looks too obvious that it's been shot in direct sun outdoors as opposed to a studio indoors and simply makes for a very poor image.
Hopefully these suggestions are helpful for those finding themselves frustrated with the "Poor background removal" refusal. They apply no matter what your subject is, whether it's food, people, objects, vehicles, etc.
Check that your backgrounds really do look clean, fresh and free from leftover pixels, and make sure your subject's edges look realistic and don't have the sharp scissor-edge appearance. And if your image description claims to have a white background, make sure it is actually white, and not grey. Pure white carries the hex code of #FFFFFF.
Good luck and happy isolating!