"Julie Bove" wrote:

It worked for a while but then I
had to keep relighting it. At first it was perhaps every three months, then
every week then like every half an hour.

Modern gas water heaters use a sealed flame arrestor chamber for the burner.
This was mandated for those who put the WH in a garage where there may be
flamable vapors.

The flame arrestor is usually implemented as a honeycomb like vent panel under
the burner. Unfortunately, the very reason that this was implemented (garage
location) makes the solution a PITA as garages tend to be dusty and dirty. The
dirt quickly blocks the honeycomb vent and the oxygen sensor for the burner
shuts the flame down.

Clean the honeycomb vent and see if your problem goes away.