Start with the shisha bowl on top. Look for chips and the oval opening that connects to your hookah. If the mouth of the bowl is egg-shaped, the bowl may not close tightly when you sit on the hookah.
Then look for obstructions in the opening of the bowl. Sometimes the holes are closed or installed during the glass-making process. You can also get information about top shisha accessories via https://www.sheeshabuzz.com.au/category/shisha-accessories/.
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You also need to make sure the bowl is tight. You should be able to pull the hookah slightly and not move around the bowl. If it seems a little wobbly, turn it clockwise as you push it past the seal.
Take out the hookah hose and stick one end with your thumb as you blow on the other. Watch for air leaks. If air escapes, the hose has a hole in it.
Now turn off one end and aim it at the bare skin area that is not on your face (there may be a piece of rust right in your eye). You have to feel how much air is coming out. If the amount of air escaping feels weak, the hose has an obstruction.
To check the airflow on the rest of the hookah, make sure it is attached with the rod, vase, vase seal, hose, and hose gasket. You will need to remove the bowl and bowl gasket if it is on top of the stem.