Desire won't send text messages to one contact

[ipodgeenie]

This is really frustrating, but my phone won't send text messages to one particular contact. I can call this contact and receive texts from them, and I can even send one, maybe two texts in a row to them, but if I try and send more than that, it just refuses to send, no matter how many times I try.

I can send texts to everyone else in my phonebook. If I wait a week or so, I am then able to send a text (or two) to this problematic contact, but then the issue just starts again.

Does anyone know what's wrong?

Thanks