If an email you expect to see isn't showing up, it's one of these:
Your mailbox isn't connected, or needs reconnecting
Go to Email & Calendar in Settings and check your connection badge. Not connected or Needs reconnect both mean nothing sends or syncs until you fix it — select Connect or Reconnect.
A reply was sent to your own inbox, not TED
This is the most common one. TED can send email from your connected mailbox, but it doesn't read incoming mail — a reply from a seller, buyer, or vendor lands in your regular inbox, never inside TED. Check your own mailbox for the response; it won't appear on the deal or contact automatically.
The email isn't linked to the deal or contact you're looking at
An email links to a deal automatically when it's sent from that deal — today that's the only way TED emails get created, so deal-sent mail is always linked. Anything else lands on the contact's own Emails tab unlinked — use Link to deal there to attach it manually. An unlinked or unlinked-then-relinked email is never lost, just not attached to what you're viewing yet.
"Sent, but not filed"
Occasionally a send succeeds but TED fails to save its own copy — you may see "Sent, but not filed — TED couldn't save a copy, so the reply-wait won't fire. Don't re-send." The email genuinely went out; only the copy on the deal is missing. If this happens inside an automator step, see Needs attention on a run for the retry options.