Check that they are not in your spam folder and also check that you have not blocked the email addresses or the email domain of the email addresses.
Log onto your BTMail account and then click on your username at the right hand side.
Select "Settings" then go to the left hand side and select "Mail". Scroll down and click on "Blocked Senders". Blocked domains are shown first then blocked email addresses. Scroll down the list until you find the one you want to unblock and select it. Click on "delete".
You should also add any known email addresses that are not getting through to you to your "Safe Senders" list.