connectivity problem between outlook for mac and my server

My emails are taking taking hours at times to transmit from bt server to outlook for mac - until recently when I opened mac outlook I would get a message saying something like "getting list of messages from bt" and the emails would appear, but this message has now disappeared - have checked the "in" and out "accounts".  on the Tools menu but everything seems OK - am a complete IT amateur and in lay terms the mac outlook doesn't seem to be fetching the emails from the server when I open up.   However, I've noticed that if I have mac outlook open and a new email comes in, this will arrive Ok  with the outstanding back mail, but if no new mail comes in while I have outlook open, I'm not getting my outstanding messages from the server.  Can anyone explain in lay terms what might be wrong and advise what I might  do?