If your email conversations are splitting into different threads, first check your Gmail settings to make sure that you have Conversation View turned on.
Click the gear icon in the top right corner of Gmail and select Settings from the dropdown. Scroll until you see Conversation View, and make sure you've clicked the button next to Conversation view on.
If you've checked this and you're still having issues, please send a message to our support team. Please answer the following questions:
Is a new thread created every time you reply with Mixmax, or just sometimes?
If it only happens sometimes, are there other patterns that you notice when it happens?
We'll help you troubleshoot!