Half of all weak-suction calls end at the grease filter — hold it up to a light and the answer is written in what you can't see through.

Where the airflow went (the fan isn't slacking — the road got eaten)

The principle is adorably simple: the blower makes negative pressure inside the canopy, and air queues through the grease filter → fan → duct → outside damper to move from indoors to out. Grease up any link of that chain and the fan still turns — but it moves a whisper of air. The filter is the first gate on that road — and the one grease attacks hardest.

Match the evidence

• Fan runs, smoke hangs, filter hasn't seen water in months → Grease-choked mesh: pull the filter and hold it to a light — if you can't see through it, that's your answer. Hot soapy soak with baking soda for 30 minutes, soft brush, air-dry, refit.
• Filter freshly washed and still weak — go look at the outside vent → Stuck damper: the one-way flaps get glued half-open by grease, and half-open means half the airflow. Power off, work the flaps, wipe the slime out.
• Recirculating hood (no duct to outside), in use for months → Exhausted charcoal cartridge: charcoal soaks up smells, not grease — it cannot be washed, only replaced, about every six months of normal cooking.

One honest limit: clean filter, free damper, fresh charcoal, still weak — listen: muffled or stuck on one speed points at the motor or its start capacitor; nothing at all points at a switch. Grease-packed ducts with too many bends belong to a duct cleaner, not an appliance tech. And one possibility isn't a fault at all: the hood's rated airflow was never enough for that cooktop.