When you seek support in a peer support forum, the people replying to you know exactly what you tell them about your issue. Do not be disappointed with the standard of replies you get, because to offer you anything one would have to outright guess. You have offered no information at all beyond "sometimes, some emails don't display correctly". There are a lot of reasons that could occur and which applies here is anyone's guess.

Email from whom? Do they all come from a single sender? Perhaps from a single email domain? Are they images that are loaded via scripts, something Thunderbird does not run for security reasons. Are the emails malformed? A very common situation these days. The standard of email developer appears to be going down.