DIAGNOSIS The brake band is not being pulled tight around the clutch drum. The linkage between the front handle and the brake mechanism is disconnected or broken.
FIX Remove the side cover. Visually inspect the metal brake band and the spring that connects it to the handle linkage. The spring should be attached at both ends. If the spring is detached or broken, replace it. Check that the linkage rod from the handle moves when you engage the brake.
IF THAT DIDN'T WORK The inertia-activated feature may be stuck. The mechanism inside the flywheel cover must move freely. Gently tap the front of the saw with your hand to see if the brake engages automatically. If it does, linkage is the issue. If not, the internal inertia springs may be faulty.
HEADS UP Do not use the saw until the chain brake is functional. It is a critical safety device. Test the brake by running the saw at idle and pushing the front handle forward. The chain should stop immediately.