This usually happens because the size of the email created exceeds 110 kb. Try to keep the size of your email template file within this limit. Every decent template will fit in this size. But if you decide to create a huge email you may want to keep in mind that some email clients, like Gmail, will display the first 110 kb of your email and clip off the rest. To see the rest you will need to click on a button.
We recommend reading this great post from MailChimp on how to best optimize your email: Gmail is clipping my email.