We hear this all the time and sometimes it's totally unclear why an image has been declined, although more often than not on P4me there is a note to explain. On some POD's such as 500px you simply get a standard email which says your image has not been 'licensed' as there are 'issues' which they can't specify, but there is a list of potential things to look at.
These 'issues' are almost always simple ones that are avoidable and stem mostly from CBBS. (Can't Be Bothered Syndrome)
Some people especially those very new to photography have CBBS because they haven't yet established a workflow. A workflow is nothing more than a set of habits that you carry out after taking photos and while some say they are unnecessary, their own way of doing things usually falls right slap bang into a specific type of workflow.
Everyone develops their own to suit themselves, but for those who don't know where to start or those who have never heard of this, here's mine as an example.
Import and First Culling
I use Lightroom to transfer my images to computer and I keyword at that point with broad, general keywords to suit the shots. Once imported I quickly scan them and mark for deletion those that are out of focus, so poorly exposed as to be unrescuable, very badly composed to the point they can't be fixed, or where something is in the background that I hadn't noticed and it would take a huge amount of work to fix. You have to be harsh with this initial cull, take no prisoners, if in doubt chuck it out.
The second cull is most important: if I spot more obvious poor ones they are marked for deletion, sometimes at second glance I see that what I thought was a 'possible' would in fact entail lots of work for not much result, I also might see one that I think is definitely a good shot, I mark that in purple. At the end of this cull I have some marked in red for deletion, some marked in purple for definite processing, and some marked in green which I need to think harder about later on. I might end up here with 30 shots from 100 taken, only 8 of which are purple. This process focusses the mind but you have to be harsh. Imagine you're picking shots for a nation wide competition and you know they have to be great.
The first process is the purple ones that I see are good shots. The very first thing I check is overall exposure and white balance, and then I check they're level. If I need to crop I do that now so I don't waste time on part of the image that won't be kept.
Once that's done the image looks better already so I check for dust bunnies, enlarging the image to 1:1 on screen for this process. When dust bunny hunt and fix is over I remain at 1:1 and check the image for things that may spoil it: a car bumper intruding in the corner, a pylon cable crossing the scene, perhaps a large piece of litter on an otherwise clean piece of grass. One by one I fix these issues and the key here is not to hurry to finish, it's to make sure you're thorough as you go. I may at this stage change my mind if an image is too much work to fix and simply put it to one side or delete it.
I've now got a set of very nice images that are correctly exposed, level, clear of dust bunnies and clean of extraneous intrusions. Now I can check the colour balance, altering maybe some yellow, saturating the blue sky slightly maybe, or toning down the red in something to stop it being garish. With a scene that includes a field of crop perhaps you may indeed want to turn up the saturation of yellow and green for corn. I should end up with an image clean of dust bunnies and other distractions, levelled, with good white balance, good dynamic range of light and balanced colours.
Now I've got a great image, I sharpen if need be using a mask for only the main edges, I check for noise especially in shadow areas, areas where I have increased exposure and areas where I have saturated the colour. I'm still in 1:1 view so as I check I also keep an eye out for chromatic aberration at obvious points, where distinct edges of highly different contrasts meet. I then go back to standard view where the image is equivalent to a 10 x 8 and view it from a distance to get an overall feel for it.
Once I am happy with the final image I think of a title for it. The title should be very descriptive, so "Ashness bridge in moonlight" rather than "The moon reflects summer warmth onto a cold night scene" which doesn't really tell us what or where the image is and for selling online it's important that people find images by what they are and where they're located.
Then I write a description. You can get slightly more prosaic here or even poetic but it's best still to include the name again and location. "The moon shines brightly on the Lake District National Park bringing into muted relief the ancient stones of Ashness Bridge, the waters beneath it tumbling chaotically down to Derwentwater near Keswick."
Now I can export the image as a full size JPG, give it a final check and upload.
Your description and keyword are the only things that put your image on the shelf. Whether they're chosen to buy or not is down to your workflow in creating a nice image. Try setting out the major parts of processing an image and make yourself a workflow and give yourself a chance to get used to it. It really does make a difference.