Jessica, I would expand on Jim's answer - from the clients perspective massage is ineffective when you have failed to identify and meet the needs, wantsand expectations of the client. So Jim's example fits this pretty well. Important to first identify, and then meet the client i.e. youhave to do enough of a pre-massage history/discovery to figure out what the client wants/needs.
A further subtlety is if the client wants something that is contra-indicated, or if the massage is actually effective in helping the client - but the client doesn't perceive it that way. Both of these are debatable at length. Part of the solution to this is in the pre-massage(including your literature) you set the correct expectation with the client.
I would say you need to tell the client what you are going to do and why, and also how you think it will be effective, get their agreement, and then deliver on that - sounds simple - it can actually be very difficult - but it is one of the keys to client retention.
This is not to say that you are inflexible - you need to be very flexible within your range of skills and use them to meet the clients needs, but you also need to be humble enough to recognise when there is no real possibility of an overlap of your skills and the clients wants and needs, and be prepared to refer them to someone else.
Sorry for the long philosophical answer - like most simple questions it was very thought provoking (thank you for that) and Idont thinkthere isa simple answer. If you want we can take it to a deeper level again!
Hope that helps and doesn't confuse
InTouch
P.S. if you want to consider a real example - say Jim's client has osteoporosis - what then is an in-effective treatment? and how do you as a therapist prevent giving it?