So email works sometimes? I guess that means it's configured correctly.
Could it be that in these cases you're sending a large attachment, and your mail server is rejecting it? Can you see the email in your Sent folder? Is it sitting in your Outbox still trying to send?
Could it be that a spam filter is blocking it? That could be happening at your end or at the recipient's end.
Do you have a cellular connection, or only WiFi? Could it be that you've left the WiFi area before the email finished sending?
Do you know what type of connection you have to your mail server? If it's MAPI or ActiveSync then seeing the item in the Sent folder should mean it has successfully got as far as your mail server. Not sure about POP connections.
It might be useful in cases where the email has failed, to try sending it to yourself and check if it arrives. That might help you to find out whether the problem is something to do with the size of the email, the contents of it, or a mail server problem.