This could be due to a high flue draught in which case you need to speak to your installer.
Doors/air controls - If the doors/air controls are not sealing correctly then this will allow extra air intake into the stove (DEFRA stoves have an air control that will not fully shut down). You can test the air controls by having the fire running, shut down all air controls and make sure door(s) are shut. Then run either a lit candle (taper) or smoking paper around to see if any smoke is pulled in. To check the tightness of the door place a piece of paper between the door and the stove body with the door shut. If you can feel a resistance when pulling the paper then the seal is fine.
If the smoke is pulled in around the doors then check that the rope and the glass gasket do not need changing (these can be purchased on our website). If the rope and gaskets are fine then either the door can be adjusted via the hinges or via the door handle assembly. Please refer to Hunter Stoves Group Youtube channel for videos on how to do this on the link below;
https://www.youtube.com/channel/UCa9zZpvA5ICwLne-JeKM1vw/videos
If the smoke is being pulled in around the top air controls and the stove is a DEFRA approved appliance and your are NOT in a smoke control zone then this can be adjusted so that it will close completely. If the appliance is not a DEFRA approved stove then you need to check that the slider plate is not warped and the gasket (not on all stoves) has not disintegrated.
If you are still having problems then please speak to the company that you purchased the stove through or your installer for them to take a further look at your stove.