Because the airflow under the deck has stopped moving material. Everything on the list below is a version of that.
The five causes, in the order to check them
1. A packed deck. Grass builds into a dense mat on the underside, usually after a wet cut, and it blocks the path the clippings travel. This is the most common cause and the least often looked at. Disconnect the machine, tip it the way its manual says, and scrape.
2. Too much material. Grass that has grown too long produces more than any deck can move. The fix is to raise the height and take it in two passes rather than to blame the machine.
3. Wet grass. Heavy, sticky, and it lies flat instead of standing up. Clumps in wet conditions are normal and not a fault.
4. A dull blade. A blunt edge tears, and torn material is ragged, longer than it should be, and harder to move. It also means the blade is doing more pushing than cutting, which slows it and costs the lift.
5. A full or badly seated catcher. A full bag stops the air escaping, so the deck backs up. A bag that never seats properly leaks at the join on every pass and leaves a line down the lawn.
The one that is not a fault
A row along the edge of each pass on a side-discharge machine. That is what the chute does; the next pass picks it up if the overlap is right.
What to do about it long term
Mow more often, so there is less material each time. Keep the blade sharp — it is a consumable, not a permanent part. Clean the deck after wet cuts rather than at the end of the season.
How this site handles it
The blade shelf here is one of the larger families on the site precisely because it is a consumable. Every blade page names the machines the maker names and states the length, because a blade that is the wrong length is a different problem from a blade that is dull.
The questions that keep coming in
Will a new blade fix it?
It fixes one of the five causes and it is the cheapest to try, so it is a reasonable first move. If the deck is packed underneath, the new blade will leave clumps too.
Is a spilling catcher the mower’s fault?
Sometimes a fit problem and sometimes a full one. A catcher that spills at the join every time, even when empty, is not seated; one that spills after ten minutes is doing what a full bag does.
Last reviewed 26 September 2026