OK. I had that problem when we first put the stove in, using an old stovepipe system we inherited from a former owner.
My son-in-law's brother cleaned the chimney for us, and found that the spark arrester screen was clogged with soot. That's common because the metal is coldest up at the top. We simply cut out the screen, since the stove pipe is three stories up and there are no woods or dry brush anywhere near the house if a stray spark ever manages to get near the ground--which I have never seen happen. No problems with clogging at the top since then.
Whether or not you want to do that depends on your own choice and judgment. Is there anything nearby that might get set on fire if an unusually large spark got out?
Oh, and I should mention that we have a metal roof. You might not want to do that if you have a cedar shake roof or something else that is easily flamable.
Just an additional thought.. Depending on the type of flue cap you have, you might need to keep in mind that a screen spark arrestor also serves to keep critters (squirrels and sparrows come to mind) from taking up residence inside your chimney during that part of the year when you aren't blowing smoke up it...
So you might need to replace the screen with a suitable critter stopper ;-)