My iPad only downloads some emails and not others. Outlook on my laptop downloads everything. help.

FFor some time now my iPad mail has only been downloading some of my emails from the BigPond server. outlook on my laptop downloads everything. My iPad is running the most upto date version. I've made sure that I have 'Leave a copy on the server' ticked in outlook and in my iPad mail. Its driving me crazy and makes my iPad mail almost useless.