Hi,
One thing to check is your folder synchronization. Go to Settings > Manage accounts > Select your account > Folders >
Tap on the Sent folder and select the option to sync it with the respective folder on the server (this may already be the chosen option, re-select it if so) > Refresh the account.
In any case if this does not help, please send us a few screenshots of the messages that are left in the Drafts alongside with the device, app and Android specifications at support@aqua-mail.com.
One more thing, does this happen for all your accounts or just your Gmail?
