Dad’s short answer: Four causes, in order of likelihood: a full bin or bag, a clogged filter, a blockage in the hose or wand, or a brush roll wrapped with hair. Three of those cost nothing and take about five minutes.

Work them in this order

Bin or bag first. A bagless bin loses performance well before it looks full, because debris starts blocking the airflow path inside. A bag two-thirds full is already restricting.

Filter second, and this is the big one. A clogged filter is the single most common cause of a vacuum that seems to have died, across every category I test. Pull it, hold it up to a light, tap it out or wash it if it's washable. Bone dry before it goes back.

Blockage third. Detach the hose and wand and look through them at a light. A sock, a big piece of debris or a lint plug at a bend kills suction while the machine sounds almost normal. Most machines have a quick-release point to get in there.

Brush roll fourth. Hair wrapped tight stops the roller turning properly, and on carpet a roller that isn't spinning means you're relying on suction alone.

When it isn't any of those

Check the seals and the bin latch. If a bin isn't seated properly or a gasket has perished, the machine draws air from the wrong place and never develops proper suction at the head.

On a cordless, a battery that can't deliver peak current under load feels exactly like lost suction on boost while seeming fine on eco. That's a battery symptom, not a motor one.

If you've done all of that and it's still weak, then you may be looking at a motor or hardware fault — but genuinely, in my experience it's a filter far more often than it's anything serious.

Dad’s Take: It's the filter. It's almost always the filter. Check that before you buy anything.