With this arrangement it doesn't surprise me the floss pads are getting clogged and the sponges cleaner. I'm Assuming in a Sun Sun the water moves from the bottom of the canister towards the top like in a fluval and the media baskets are stacked one on top the other.
If so, I'd switch the filter sponges to the bottom .. they have larger pore openings to let them trap more of the larger particles ... then your filter floss pads to trap the smaller particles and finally the ceramic media. Keeping the floss pads at the bottom will quickly clog as they'll trap practically everything reducing flow eventually.
A good alternative to the floss pads is 100% (no fire-retardants, dye's etc) Poly-fill you can buy at any arts / craft store, walmart, kmart etc. LFS
do also see them ... albeit over priced. Go with one of the others stores. A 20oz bag (I use fairfield brand) for $5 -$6 can last you months. I used to run a Fluval 404 for my 75gal and would stuff two of the media baskets with polyfill. I still use them in all my Aqua Clears.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.