The vacuum isn't broken — it's choking on its own trophies. A dust bag two-thirds full already strangles the airflow; and a machine that sounds normal, or even louder, is usually leaking air, not gaining suction. Walk the airflow path once, and the break shows itself.

Where the airflow goes (suction is a sealed-system business)

A vacuum doesn't "suck" — the motor pumps the inside down to low pressure, and outside air pushes the dirt in. The route is one-way: floor head → hose → dust collection → filter → motor → exhaust. Choke any link and everything downstream goes limp. So lost suction has exactly three accounts: the route is blocked (sudden), the filter is saturated (gradual), or the bin is so full the air can't pay its toll (gradual). Suction is a sealed-system business — wherever the crack leaks, the suction dies.

Match the evidence

• Sudden loss — it died right after vacuuming up something → Blocked airway: unplug, detach the hose, look through it with a flashlight, or drop a coin in one end and see if it rolls out — socks, paper wads and hair clumps love the first bend. Push them out with a broom handle.
• A slow fade over weeks → Check collection first, then the filter: a bag at two-thirds full already chokes airflow (fine dust packs the bag's pores — don't wait until it "looks full"); if the bin's freshly emptied and it's still weak, it's the filter — hold it to the light, and if light won't pass, rinse with cool water and let it dry for 24 hours before it goes back in.
• Fine on hard floor, weak on carpet → Not a suction problem at all — it's the brush roll: a cocoon of hair slows the roller and kills the carpet agitation; a stretched slipping belt usually announces itself with a burnt-rubber smell.

Two counterintuitive facts: sounding normal, or even louder, ≠ the suction is still there — that's the sound of a leak, air entering through cracks and loose fittings instead of through the carpet; run your hand the length of the hose and the hissing spot is your break. And a cord left wound on the machine heats up and drops power — unwind it before you judge. One honest limit: hose clear, bag new, filter clean, seals seated — still weak — the motor's carbon brushes are worn out or the fan blades are chipped; a tech comes with parts, and on an older machine the bill may rival a new one, so do the math first.