Sometimes you need to turn off and back on the mobile devices to get them to update, and I think in my phone I have it set to only hold a certain volume of mail, so if I get attachments, they gum up the works and won't let more download. It's set to show only two weeks worth and hold a certain volume - if it isn't time for the big ones to drop off of the list, nothing new can be added.

You might have to sort and purge on the mobile device.