Why won't my mms texts send?

I had this problem as well and fixed it by tinkering with a few settings (during which time I realized there could be any one of about 10 reasons this might happen). My best suggestion is to complete all of the following steps in order, (it's easier than it might sound by the way I wrote this, so don't get discouraged) :

Go through to main settings menu (slide your finger downward from the top of the screen), on the Wireless and Networks part of the setting menu, select the "More" option and then choose Mobile Networks to first ensure that your Data Enabled and Data Roaming options are both checkmarked (because even if you are on a WiFi connection, it still requires the use of your data plan for MMS type texts).

Next, in the same Network Settings menu screen, select the option "Access Point Names", then once on that screen, touch the settings option in the upper right corner (it will either be 3 vertically aligned dots or a 'gear' looking icon). Select "Reset to Default" just in case something got "outta whack" in the Access Point. FYI mine happened to be wrong one time because I had changed my SIM card; when I chose 'reset to default' it changed it to TFWAP, for Tracfone and fixed the MMS issue I was having that time (I've had more than one MMS issue before). Next go back to the previous menu and select the option below the "Access Point Names", titled, "Network Operators" then select "Search Automatically" and wait a second for it to say something like "registered on network". You can then close the entire 'main settings' screen.

Lastly, open your "Messaging/Messenger" application and ensure you are on the main screen which would show all of the people you are sending/receiving messages with. While on this screen, press the Square shaped 'button' at the bottom of your phone (next to the 'home' and 'back' buttons). It should bring up a menu with a few options such as: "Delete all threads", "Settings", "Configuration Message", "Wap Push Messages", "Wireless Emergency Alerts", and "Mark all messages as read". You want to select "Configuration Messages".

Once you're on that screen you should see at least 2 options or more that say, "Configuration message: application, Browser, access point..." and another that will say "Configuration message: MMS, access point name...".

You want to open both but do one at a time first by clicking/tapping the one that said "application: Browser, access point" (even if it already said "installed" before tapping it). The configuration screen might already show some information on it but don't worry about that. Just select "Full Install", a confirmation will pop up saying that "current settings will be overwritten", ignore it and continue and another pop up will confirm that it was successful. Now select the one that says "application:MMS, access point name" and follow the same instructions as you did for the previous one.

Next, close all open apps and restart your phone by holding down the "Power/Sleep" button and selecting Restart Phone.

If there are things that were missing or didn't match up with what I've explained above, or if it doesn't solve the problem, then you'll have to make the painful call to tech support for your carrier/provider. Good luck!