It’s common to see masonry debris in the base of the appliance liner which can lead to an obstruction. A trained professional will always take apart the vent connections and look into the chimney to see if there are visible signs of deterioration.
Often when having a new natural gas or propane water heater installed, a new chimney liner is recommended, or even mandated by code. This requirement is a critical safety consideration for your property and addresses several issues.
Information provided by Lindemann Chimney and Fireplace 2018