There are probably a myriad of reasons why networks behave the way they do but here is a tip I stumbled onto that might help if you are having trouble sending pictures via MMS on your Nokia Lumia 920.
Go into your settings and tap "access point". Its on the bottom of the list.
Then, once inside there are settings for
AT&T - LTE 1,- Lumia
AT&T - LTE 2 - Lumia
and AT&T 3G.
Make sure you are attached to the correct band otherwise you will have trouble with your LTE network. Mine was set to LTE 1 and I could not send pictures via MMS. I kept getting this "unable to send" error. Once I changed it to LTE 2, I had no trouble sending pic via MMS. The AT&T support center wanted to hard reset my phone "hoping" that would resolve the problem. It probably would not and I then have to redo my screen again. (A non starter) Changing this setting also improved my overall network experience as a bonus.
Go into your settings and tap "access point". Its on the bottom of the list.
Then, once inside there are settings for
AT&T - LTE 1,- Lumia
AT&T - LTE 2 - Lumia
and AT&T 3G.
Make sure you are attached to the correct band otherwise you will have trouble with your LTE network. Mine was set to LTE 1 and I could not send pictures via MMS. I kept getting this "unable to send" error. Once I changed it to LTE 2, I had no trouble sending pic via MMS. The AT&T support center wanted to hard reset my phone "hoping" that would resolve the problem. It probably would not and I then have to redo my screen again. (A non starter) Changing this setting also improved my overall network experience as a bonus.