Why is it that I can send but not receive mms on my Sony Xperia Z1 compact?

[AC Question]

I can send but not receive mms on my Sony xperia z1 compact

My provider is Straight Talk, and I have gone through all the settings with them several times, after which they told me the problem isn't on their end. Any suggestions? (I am going to contact Sony tomorrow, but I'd like to get some of your ideas also.)