I have a Dyson DC07 that has been losing suction over time. I had tried cleaning out the holes in the canister and making sure that all the hoses were clear, but still the suction was really bad, to the point where my vacuum was hardly picking up any dirt.

The thing that finally fixed my Dyson DC07 was cleaning the pre-motor filter which lives to the side on the base of the vacuum. It's basically a piece of sponge that lives to the side of the motor. Mine was filthy. I washed it in the sink with running water until it was clean, it took about 10 minutes to get the water running out of it cleanly. I let the sponge dry over night and then put it back in. Man, what a difference. My Dyson is back to full suction now.

After cleaning the filter out, I could see that it says right on it that it's supposed to be cleaned every 6 months. I can tell you, keeping this clean makes all the difference in keeping your Dyson working well.