According to the repairman; our propane water heater does not draw properly - how can we fix it?

How is it installed? It should have a double-walled flue pipe installed on it and if next to house the pipe should be at least two feet higher than any part of the house within 10 feet, but three feet is better and required in some places. The heater needs to be shielded from any wind, but have plenty of fresh air for combustion. If it's installed with these requirements it should draw okay.