fluorescent bar light in kitchen

Hello all, Our 2 fluorescent bar lights in the kitchen burned out after 4 years so I replaced them w/ new bar lights.
I must not have replaced them correctly as we see it takes a bit longer for them to light up and several days ago, it would light up dimly occasionally. Since then, I took them out and put them back on, and now, they light up although it takes longer than the original bar lights.
The question is: These bar lights have 2 prongs at both ends. I slide them into the fixture, but how do I know it's locked in correctly?
Sorry for the newbie question.
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