It’s quite easy to tell when your Dyson has lost suction; it will struggle to pick up debris from the floor, perhaps accompanied by a strange noise. Using your Dyson in such a state will be counterproductive, so you need to find the source of this problem. There are several factors that can contribute. The most obvious reason is that your canister is full — if you’re not used to using cordless models, this can happen faster than you know. Take a look at your canister and empty it if necessary.
A hidden blockage could also cause your Dyson to lose suction. Take your vacuum apart piece-by-piece, inspecting each until you find the location of the blockage. If it’s in the hose, which is pretty common, you will have to fish it out with something long and thin, or otherwise soak this part.
A dirty filter can reduce the suction of your Dyson as well. This is because it can no longer vent the air through as efficiently. For guidance on how to clean this, follow steps 4-5 above. Dyson recommends cleaning its vacuum filters once per month, but you will need to clean it as often as required depending on use. Some of the more advanced models will even stop running and inform you if the filter needs to be cleaned.