There are two different reasons that could explain these symptoms.
Sometimes the automatic settings work well for call, SMS and Internet but were unable to modify the parameters for MMS. It's typically the case when you can neither send nor receive MMS.
You can change the MMS settings manually, the correct settings are:
If you need more information on how to change these settings, you can search for your particular phone model here.
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If you are able to receive MMS but not send them, your settings are correct but you may have an insufficient balance to complete the operation.
MMS are not included in the goodybags allowance and they cost 16p each. If you don't have credit in your balance, you will need to top up to be able to send MMS.
You can find more information about topping up in this article.