Suction is not so powerful - Dyson DC05 Cylinder Bagless Canister Cyclonic Vacuum

Yes, in one way the suction would be higher, but there would now be an additional load on the motor. Also, it would have more suction, but less volume (air flow).

If there is no suction at all, not even a little bit, then most likely the suction fan is broken. They aren't too complicated to replace if you want to.

if there's no blockages and filters all OK it's probably the suction fan that as worn down and not creating as much suction..

It automatically switches over when you disconnect the hose from the power nozzle. No suction it getting to the power nozzle, so the suction is only through the attachments. If there is no suction through the hose, check for clogs in the hose and in the tank where the hose plugs in. Also, check the screen at the bottom of the tank. If there is a clog in the hose or tank, then there also will be no suction to the power nozzle.
Remove all clogs and you should get suction.

You need to replace the belt that turns the brush.

Check your belt. It is broken or stretched out. Replace the belt first. Should be your answer

Disconnect the suction hose. While running, check the suction port if it works, by putting a piece of paper. If the paper is taken in, then it is oK. Now, check the suction Hose for breaks or opening before the suction head. Breaks or opening breaks the suction power at the suction head. Loose connection between the suction pipe and the suction port also must be check as this will also cause the same
problem. You may need to replace the suction hose
to correct this problem,
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vax power pet 1 filter keeps getting clogged up so dirt not going into cylinder

You say it lost its suction, then you also say this is plenty of suction power, seems you cant figure out the prob. Try cleaning the fan on the suction blower. The blades can get chocked with dust which will give you a reduced suction.

Have you checked the filters?