All 5.0kW output stoves and above installed in houses built before 2008 are legally required to have a dedicated air supply or 'air-brick' installed at the same time. In post 2008 houses then all stoves, whatever their output, require a dedicated air supply. This is an essential safety requirement and there's no getting around this.
If your stove was already installed when you moved into your home then you should check that you have an air supply suitable for the stove's output. Some installers calculate that pre-existing air-bricks are sufficient to meet the relevant air supply requirements and therefore have not created a new 'air-brick'. Some installers just simply don't consider this apre-requisite for a stove's safe and efficient operation. However, if you believe that your stove's output requires an air-brick and you can't locate one it is worth considering whether your general stove installation needs to be surveyed to determine if it meets current safety standards.
If you have an air supply, check that any air-brick or vent is clear and continues to allow the free movement of air. Check the outside condition as well to ensure that it has not been accidentally covered up or that weeds, foliage or wind-blown debris have not blocked it.
For stoves which feature a Direct Air supply (also known as Outside Air or External Air supply) disconnect the pipe at the rear of the stove at the vent end and check this for potential blockages. Use a vacuum cleaner to clear this if necessary. Check that the outside grille has not been accidentally covered up or that weeds, foliage or wind-blown debris have notblocked it.