I have MMS turned on on my phone, but it seems like longer texts (possibly greater than the 160-character limit) rarely - maybe never - reach my girlfriend while she'll get shorter messages with no problem (my phone seems to suggest that the messages go through, though). Occasionally, I won't get her messages either, though I'm not sure of the length of those messages. She has an Android through a different provider, so I'm not sure if that is the issue or if it's related to my phone. Anyone else ever have these problems?
It probably has more to do with the carrier to carrier interaction with long messages. Many carriers simply either do not know how to handle long messages or do not have their networks set up to correctly handle them.
I have seen several posts over the years regarding this exact issue. It is rarely if ever an issue on the same carrier. Add a second carrier to the mix and all bets are off.