I'm with Dan on that. The only other possibility is the cartridge. On some 901 carts the floor of the cart well is not well fixed and can move sometimes blocking the flow. (Usually on 'D style carts where the top looks like a D and there is just one air channel to the mouth piece.)
When I detect a hard draw I'll take the cart and check it, and then 'tail pipe' the atty on the batt - drawing over the atty chamber without a cart. It will be one or the other or both, including what Dan says on the hole in the atty body.
I guess there could be a case where there is so much gunk in the coil area for the air channel through there to be obstructed, in which case, clean the atomizer.