We're just not going to tell you.
Call your carrier perhaps, if you're worthy, they might tell you why.
He's a clue... if you want something fixed, you have to tell us something other than "It's not working"
Would you expect your mechanic to know why your car doesn't run if that's all you tell them is that "it doesn't run right"?
Are you that thick in the skull that you think there's only one possible reason that it's not working, and someone out there must know why?
I wasted my time by telling to you call your carriers customer service, but I'm sure you can read, right?
You waste your own time.
I just purchased a Moto Q phone and had it activated with Cricket. How come I can receive pictures but can't send any out?
Did you request that the internet be blocked to make your monthly service cheaper?
To send/receive you not only have to have medianet (at least pay-per-use) provisioned on your account, but you must have it registered. (only happens automatically if you have the SOC provisioned at time of activation)
I'd call customer care at either 611 from your AT&T phone or 1-800-331-0500 from another phone.
Just let them know your MMS isn't working.