Almost all cases of get smoke back into the house are due to leaks in the exhaust system (chimney) and these can be a bit tricky to find because you can smell them but not see them.
This is what you do to find where the smoke is coming from. Start the stove normally. Get a decent fire going in it and then take a big handful or cup of pellets and throw it on the fire so that the fire goes out, smothering the fire. Close the door to the stove and crank it up to high. You will get a lot of smoke coming from the pellets as the try to ignite. Turn on your flashlight and look around the back of the stove and you should be able to see smoke whisping out of where ever your leak is. From there you can use metal tape or silicone to seal the leak.