How strange! I've seen content being stripped from emails a number of times before, and it usually comes down to one of a few things:
1) Corporate level Spam Filters/Mail Servers stripping or blocking content from displaying
2) Coding issues that a particular email browser isn't able to render correctly
3) Lead not meeting dynamic content requirements - so no content displays
4) Gmail using conversation view to consolidate content within an email that's been received before
Was the address that received the email lacking content an internal or external address? Was it a corporate or freemail address? Was their dynamic content within the email?
Hope that helps