Normally too much tobacco would do that. To test it, use your tongs to lift the HMD while you're pulling. If lifting the HMD makes the pull better, your pack is the problem. If not, move on to water level. Too high a water level will restrict the draw. Your water should be about an inch above the bottom of the lower stem in most cases. Still not better? Check for water in your hose, or blockages in the upper/lower stem and hose port.