I think we would really need to have some more information to figure out why this is happening. If they are correctly guessing the method, then it most likely is that you need more practice and to be more aware of you angles. If that are incorrectly guessing the method, it most likely is your presentation.
Even though you may not be presenting your magic as a challenge, your presentation may lead the audience to focus on the method. Let me explain. If your patter is narrating what you are doing in a say-do-see manner (saying what you are going to do, doing it and then telling the audience to see) the audience is going to focus on the effect (result) and then focus on the method (your actions). The reason they do this is because all you talked about was what you were doing (actions) and then the effect (result).
Other things may also affect your spectator's perception. Sometimes we as magicians try to over prove some things. "Here is the top card of the deck, it's only one card and I'm going to put the one card back on the deck." OK, that's an exaggeration, but you get the idea. Additionally, sometimes we inadvertently telegraph the method through what we say and how we act. We tense up at the moment of a sleight or look at our hands or stop talking or even just hesitate a second -- the audience's attention is drawn to those moments.