LaTeX cards render ok on deskotp, some fail on web and mobile

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I’ve built a deck of cards with LaTeX symbols I want to memorize, the symbols are used in the “Temporal Logic of Actions” or “TLA+” software specification language. On each card’s front I display some LaTeX symbol, for example the Greek capital letter Phi:

I accomplish this by entering a LaTeX snippet into one of my card’s fields:

[$$]\Phi[/$$]

This works well on Anki Desktop, but on mobile it looks like a black card with a gray square where the Phi symbol should be.

On the web, the symbol appears as a “broken image” icon.

Most LaTeX symbols in my deck do work correctly on all platforms! It seems unpredictable which will work and which won’t. For example, the “much less than” symbol works correctly with this LaTeX source:

[$$]\ll[/$$]

That renders correctly on all platforms. However, this “much greater than” symbol only works on Anki Desktop:

[$$]\gg[/$$]

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On mobile, the misbehaving Phi card appears like this:

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Here’s the Phi card’s appearance on the web:

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Trying using Tools>Check Media on the desktop before syncing to generate the images.
Please see https://docs.ankiweb.net/#/math?id=webmobile

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Terrific, thank you. I found that there were some LaTeX errors logged (I had a few cards with bad LaTeX syntax), and other cards were fine but somehow their media was “missing”. Once I fixed the syntax errors and regenerated all media, then synced, the mobile app works correctly.