sporadic delivery of MMS messages/messages.att.net apparently not working

Hi. I only receive sporadic mms messages on the ATT messaging app--usually it's text messages (not pictures) in group threads that don't show up. Sometimes I get a notification that my phone, an LG G3, is trying to download something but it never actually works; other times there's no indication whatsoever that I've received a response. This is extremely frustrating and I have no idea what I can do to resolve it.

On similar lines but maybe just it's just two different problems I'm having with ATT, the messages.att.net page shows a preview of my messages (including the ones that don't download) but cuts off the body of the messages about 3-4 messages before the most recent one. Pleeeeeease help, anyone?!