Suction has diminished since I purchased it a year ago. Hose is not blocked, Cannister is empty, and plastic filter above cannister is clean. What could be the problem?

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..

We have one of these a few years old without the ball, pull out and see what suction is like with just the hose and the attachment for it, am I wrong and it (yours ) does not have the pull up and out hose? low suction is almost always a blockage in the air path between motor and cannister.

check the impeller on the moter it may be missing a fin or two (question does it make any unusual noise or vibration ?)

Change or clean the filters

OK. Lost suction almost always means blockage. You need to check all the pipework, the hose and any bits you can remove at the base of the machine - if I remember right, the DC17 has a sort of U shaped bend at the bottom of the handle that you can remove - also turn it over and inspect the clear flexible plastic tube (if it's not blocked, it can get damaged and leak). As for filters, you do of course need regularly to wash the filter that's above the cannister (wash both the blue sponge and the yellow carrier) and be sure they are dried gently for at least 12 hours before putting them back.  If you need new filters, any vacuum service shop should be able to supply them. I hope this helps. I value your feedback so please remember to rate my response.  Good luck!

Yup! Clogged hose on the back of the cannister! Unit is designed to shut off for 30 minutes when this happens to protect your motor. Remove top of hose where debris enters the cannister through a filter above it, then remove the other end of the hose from the bottom of the back of the cannister, take it outside and use something long that'll fit inside the hose to force out all the clogged debris and hair through the top of the hose.

the hose from brush go to the bag need to check for blockage you have to open there hose to clean

Is there suction where the pipe hooks to the cannister? If so, then are all of the outlets without suction? Maybe the hose is plugged. If the hose is ok, and there is suction at the cannister, then **** up a small **** of paper, one for each room outlet, have a number on each piece of paper, Which ever one doesn't ,ake it thru to the cannasiter is the outlet that is plugged. You can use a plumbers snake to clean out a clogged outlet or even a clogged hose.

If the motors are running and sound ok, then there has to be suction at the outlet on the cannister. If the filter bag is old, it may be best to replace it with a new one.

Sounds like the motor is overheating and thermalling out (Hight Temp shut down). This could be caused by overfilled cannister, obstructed passageways, or debris accumulated around the motors cooling fan. Try emptying the catch container and blow out / clean all the filters. Check for obtructions by going to the hose mode and alternately blocking and uncovering the nozzle end - feel for good suction and listen for a definite change in motor sound (Pitch Change) With the intake open, the motor should rev freely, when blocked it will slow and sound labored / loaded down. Another possible cause could be a binding brush roller overloading the motor - but this will usually burn out the belt first. Notice any unusual smells? Burned belts stink like burnt rubber.