2 updated with ballast info
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I bought FEIT replacement LED tubes at Costco, item number T48/840/LED/2, and swapped them in for fluorescent tubes (the thinner style). When I switch them on, the pairs of LEDs barely illuminate. They have an faintly-imperceptible glow but are essentially off.

When I give a good tap with my fingers on the end of the fixture on just one of the tubes in a pair, the pair of bulbs immediately lights up and stays on until I switch the lights off. Then it's back to the original state.

These are supposed to be plug-and-play bulbs with no need to bypass the fixture's ballast. I can't find any identifying model markings or specs on the fixture The ballast says 120V/0.80A 50/60Hz, it's made by Keystone, part number KTEB-232RIS-1-TP-SL.

What might be the problem such that it is fixed every time by tapping the end of the fixture?

I bought FEIT replacement LED tubes at Costco, item number T48/840/LED/2, and swapped them in for fluorescent tubes (the thinner style). When I switch them on, the pairs of LEDs barely illuminate. They have an faintly-imperceptible glow but are essentially off.

When I give a good tap with my fingers on the end of the fixture on just one of the tubes in a pair, the pair of bulbs immediately lights up and stays on until I switch the lights off. Then it's back to the original state.

These are supposed to be plug-and-play bulbs with no need to bypass the fixture's ballast. I can't find any identifying model markings or specs on the fixture.

What might be the problem such that it is fixed every time by tapping the end of the fixture?

I bought FEIT replacement LED tubes at Costco, item number T48/840/LED/2, and swapped them in for fluorescent tubes (the thinner style). When I switch them on, the pairs of LEDs barely illuminate. They have an faintly-imperceptible glow but are essentially off.

When I give a good tap with my fingers on the end of the fixture on just one of the tubes in a pair, the pair of bulbs immediately lights up and stays on until I switch the lights off. Then it's back to the original state.

These are supposed to be plug-and-play bulbs with no need to bypass the fixture's ballast. The ballast says 120V/0.80A 50/60Hz, it's made by Keystone, part number KTEB-232RIS-1-TP-SL.

What might be the problem such that it is fixed every time by tapping the end of the fixture?

1
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Replaced fluorescent tubes with LED tubes, the LEDs only turn on when tapped

I bought FEIT replacement LED tubes at Costco, item number T48/840/LED/2, and swapped them in for fluorescent tubes (the thinner style). When I switch them on, the pairs of LEDs barely illuminate. They have an faintly-imperceptible glow but are essentially off.

When I give a good tap with my fingers on the end of the fixture on just one of the tubes in a pair, the pair of bulbs immediately lights up and stays on until I switch the lights off. Then it's back to the original state.

These are supposed to be plug-and-play bulbs with no need to bypass the fixture's ballast. I can't find any identifying model markings or specs on the fixture.

What might be the problem such that it is fixed every time by tapping the end of the fixture?