There are a couple of reasons why Gmail does this. Let's see what they're looking at -
Gmail will clip off all messages that exceeds 102 Kbs. This is the reason why your messages are being clipped off in 90 percentage of cases. You can use the "minify" option available in MailFed to compress the size, but remember, it won't work if your content is many times larger than 102 kbs. So, make your templates lite and simple.
If you're sending multiple emails with the same subject line, Gmail will hide the repetitive contents as the entire "thread" is cascaded with the same subject emails. This will appear different though. So you'll be able to expand the content there itself by clicking the "three dots" in the bottom of that message.
If you are copy-pastig the content from some other website, it's possible that some code/style section can also be copied along and that might cause issues with Gmail because these code sections may be blocked/not allowed in emails. So, when you're copy pasting the contents, either paste it to a notepad first or use the "source" section of the editor to make sure no illegal code sections are accidentally copied.
Lastly, if there's a lot of content, that also trigger the Gmail filters to clip off your content. See our email designing principlessection to know how to design a better email template.