It might be worth mentioning as well that sometimes when you forward an email, say to a colleague or reviewer instead of sending a direct email you might also experience this issue. This might account for why you're having difficulty recreating the issue as well? Usually by the time an email lands in front of an executive teammate it has been reviewed internally and most times the one that looks good is the same one that gets forwarded along for them to review. In these cases, the whole thing gets sorted out with a direct send (as it would be to your audience).
As I understand it, you can initially code an email to speak "many languages" - there's a little bit of Outlook, a little bit for gMail, and so on. Once the email hits your inbox - lets say it's Outlook - it'll strip away any code that it doesn't like and keep what it does. In my experience, Outlook is the biggest offender here b/c their syntax is kinda - "the way it's always been done" - if you will. Once you get an email in Outlook and send it along to a gMail account, for example, it's lacking all the gMail bits that Outlook stripped from your code. It sounds like you're missing some of the mobile styles and there are some email clients that just eject styles in the head entirely, so this could be the case as well if you're seeing this issue with forwarded emails.
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