You might encounter problems with the draw of your flue if the chimney is too short, if it has inadequate insulation, or if it is leaky. In this case, it's best to get a chimney sweep to thoroughly check your chimney and to measure the flue draught.
If the above problems have been eliminated, you can extend the lighting period of your stove to ensure that the chimney is well heated. The draw on the chimney will be greater when it is hot. This is achieved by using lots of kindling and 2-4 firelighters. After igniting the firelighters, leave the stove door ajar for 5 to 10 minutes. When the majority of the flames have died down and there is a good amount of glowing embers, you can add 2-3 pieces of dry wood and keep the air supply open.
On chimneys with slower flue draughts it might be necessary to leave the stove door slightly ajar again until the logs have fully ignited. Once a good flame has been established and the door has been closed, you can set the air control to your desired position. If you have an old brick chimney with a large inner diameter, it can be a good idea to reduce the area with a liner. To do this, you will need to seek the help of a HETAS engineer, your Hobbit stove dealer/installer or qualified chimney sweep.