She doesn't want to massage you
So you have had a small pain in your back for a while now and you keep asking your respective other half to rub the pain away and sometimes they may but more often than not the answer is a deflection, delay or outright "not now". So why is this?
Agape is the Greek derived term for a self sacrificing love. The Hawaiian 'Aloha' has a very similar meaning. The context in which this applies for massage is the emotional contract forged in the giving and receiving a massage. Within the confines of a relationship the two partners may be entering into the massage with very different expectations making it difficult for one to massage the other.
Massage practitioners are trained to offer neutral energy, neither taking from the client or giving to them. We are not the healers here, just the hands through which healing is offered. Sometimes it could just be a small back pain, and at other times it could be connected to the way you are thinking or feeling.
Receiving a massage without expectation could be the reason your partner keeps saying No! Your partners experiences of previous requests may be setting you up in the wrong direction. if you have previous asked for a massage with an outright expectation that things would go in other direction you have created a precedent that will hinder further requests.
Take the pressure off your relationship and ask someone who is neutral like a trained professional and Its probably best to invite your partner with when you do get that massage!