Keep in mind that if you're sending any media with the text (ie, photos), sending group texts, or sending emojis in your texts, that requires MMS. SMS is an ancient protocol, and just can't handle those additional components.
Sorry about the late reply, been on holiday! Carrier is UK provider '3'; Android tells me the message is being sent as MMS, that it has not been sent, then suggests sending as SMS, which I'm sometimes not quick enough to accept, and therefore no message, MMS or SMS , is sent. Message is being sent to another android phone. No signature; no image; text only. Complicated by the intended recipient not always having her phone switched on. Only happens with this person.
If the phone is having trouble sending MMS, that often suggests an issue with the mobile data -- do you have problems sending MMS to anyone else? You said it only happens with this person, but I'm not sure if that means SMS gets converted to MMS only for this person, or that MMS can't be sent only to this person.
The problem is primarily with the one phone; I have not had any problems with other contacts - but my texts generally do not include photos, and are small in size, and are rarely group texts, so I may not notice any problems with SMS being converted to MMS, and vice versa. The problem appears to be with this one phone - my phone tries to send text messages to her as MMS, but they don't go through, a message comes up to say that their is a problem and suggests sending via SMS, but there is only a short window in time for me to press the acceptance button. If I do not notice , or am not quick enough - the message is never sent, and I have to use email or WhatsApp to make contact with her