Clean all of the stove, that means every place between just before the air intake until just after the vent termination.
That is the simple description, in reality you need to use brushes, high air flow devices, and something to dislodge clumped up and stick on ash. The usual culprit is behind the firebox walls, open all access ports, and insert brush, a piece of stiff wire whatever and go left, right, up, and down, use a proper vacuum in there as well, then get back in there with the brush, you need to do likewise around and through the heat exchanger as the air flow goes around it and to the left and right and down behind the fire box wall then joins back together and heads for the combustion blower cavity, this path needs cleaning as well, then there is combustion blower cavity, after which there is yet another shorter pathway to the vent system, the air flow has to be good all the way to outside the vent termination, a lot of folks have been know to forget to clean the vent cap. Follow up with a high air flow device place all access covers.
You likely have a damper on your system so I'd make certain that it was not finding itself blocked with ash or outside any slide-way.
