I use denatured alcohol to clean the rollers on my Makita, and it doesn't seem to have harmed them over the years, but I do this infrequently. If your rollers are coated with pitch and or chips they won't feed well, no matter how they're adjusted.
(I don't think this planer has the capability to adjust the spring tension on the rollers, but I could be wrong.) You can consult the manual online at http://www.dewaltservicenet.com/Products/DocumentView.aspx?productid=22538&typeId=7605&documentId=21151
If you can attach dust collection to the planer it will probably help.
Also, the direction of the grain makes a difference when trying to plane boards as dense as poplar, (which has a kind of weedy, roey grain sometimes, that would prefer to tear out.)
For example, if you were feeding the board from right to left of the page, the grain on the side of the board should look like this//////// instead of this\\\\\\, which will have a tendency to tear out and jam the planer. If you can make sure the boards feed through in the right direction, this may solve your problems.
I heartily recommend the wax, as well. And possibly compressed air with a nozzle to blow chips aside.
Also consider the length of the boards and the effect of the drop weight on the planer's rollers if there is not an extension roller in place (or a worker) to hold the board as it makes its pass through the machine.
If you feed the board with the cup side down to begin with, taking light passes to smooth the top face, then flipping it over end over end and feeding the other side in the opposite direction, flattening first one face, then the other, you might have better luck.
Aside from all this, a surface planer isn't a jointer, which is designed to straighten & flatten boards, and portable planers in general won't perform as well as industrial models of the same capacity.
If you will continue to be running a lot of stock through your machine, you might want to consider investing in a used 13" Rockwell or Delta or some such cast iron planer, with rollers top and bottom to facilitate movement of course material. They aren't as portable, but might save time in the long run.
On a final note, there's no replacement for sharp knives. Even the best planer won't drive stock well when the knives get dull.
Best of luck!