Add-in would not remain in Outlook 365

What are some reason why an add-in would not remain in Outlook 365?
I was able to get an PSShip ship add-in to remain in Outlook by first ensuring  Microsoft .Net Framework 2.0 was installed on Windows 10, then running Windows Updates and let Windows check for updates for other Microsoft products. Afterwards, I could successfully push the PSShip add-in to the machine.
I know of a way to force Outlook to keep the add-ins, for example by modifying the registry but I am looking for the root cause.
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Thomas Zucker-ScharffSolution GuideCommented:

I have added a couple of topics to attract more experts to this question.  I will keep an eye on it in case it needs further escalation.

Tom Zucker-Scharff
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Jian An LimSolutions ArchitectCommented:
can you check vendor about the addon support?

which vendor was that?

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MAuthor Commented:
I am not sure who the PSShip  vendor is, but you gave me the solution, thank you.
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