if you cant get a hit from your bong....

1: packed the bowl too tight (just get a toothpick/hanger clear it out and re-pack)
2: youve smoked out of it enough to where the resin buildup has clogged the stem just smoke the resin or scrape it out and save/smoke it.
3: Bowls of weed dont stay lit like tabacoo just keep the fire on the weed while your sucking in.