Polyfill will start to slump in the cartridge after a while. This is because the filler sits on the metal bridge inside the atomizer. Keeping it pulled up will keep your atty wet and give you good consistent vapor. I use a large darning needle to fluff it up and bring it to the top or a bent paper clip.
Polyfill can be cleaned (as Kent directed) and refluffed but nothing beats a clean piece in a cartridge. When I was still using polyfill (I've switched over to something else), I found the best thing to do was to take one of the new cartridges (that didn't have prefilled liquid in it) out and put it in a small zip lock so I'd have it to compare to when I cut new pieces.
Sheet polyfill can be found at most craft stores (the sheet type is easier to cut and get the right size for your cartridge vs. the bag of loose polyfill). They make it for quilts (called quilt batting) and it comes in various thicknesses. You might ask if they have it on the roll and get a 1/8 or 1/4 yard cut. You may also find it in a precut bag (see if they carry a precut size for a child's quilt or wall hanging size).
Another trick that can help you with polyfill is the straw modification (you can also use a hollow plastic q-tip, cut off the cotton bud ends and use the hollow plastic... look for the colored ones to determine which are plastic. The cheaper the better and you can usually find them at a .99 cent store). The directions are here
http://www.e-cigarette-forum.com/forum/tips-tricks/11421-cart-mod-better-refilling.html This keeps air from being trapped in the bottom of the cartridge and allows for the liquid to flow better out of the cartridge.
You can also try some of the various fillers as a replacement for the polyfill. Kent mentions the blue MarineLand foam (aquarium foam), some use Buff Puffs, Lipton Pyramid Tea Bag (works in these 901 carts too) and my favorite for over 6 months
is the AquaClear Foam (aquarium foam). You can search the forum for information on these alternatives or PM me if you can't find anything and I'll send you some links. All of these are easy to clean, hold their shape in cartridges well without slumping and can be reused many, many times.
Happy 901
vaping!