Just make sure, if the the bowl is a slider, that it has a rubber ring around the lower part of the bowl to make the hits near airtight.

If you have a shotgun hole, make sure the bowl is on airtight.

That should fill your bong up with enough smoke. If the bowl area isn't airtight or near airtight when you take a hit, air goes through the bong, and it doesn't fill up with smoke.

For an 18inch bong you should be getting more than a lungfuls worth.