This has come up before and I was about ready to replace the ballast. But
others have posted that sometimes corrosion could be preventing good contact
between the contacts on the long bulb and the fixture so I worked the bulb
in and out of the fixture several times and the fixture is now fixed. No
new ballast was needed.
In this case your fix worked.
There is also another thing people should check, make sure the ballast case
has a good ground as well.
(Sand off a bit of paint/rust if needed)
Some cheaper ballasts/fixtures can be quite picky, right from new, and just
will not "start" themselves without a solid ground connection.
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