Have a look to my answer to Finding out which tile a mouse click landed inFinding out which tile a mouse click landed in. Your error here is to forget that the isometric has a different coordinate system from the one used on the screen.
You should consider to store the players coordinate position in term of isometric coordinates; the position in term of (x-tile-index-and-fractions,y-tile-index-and-franctions) may work and help your game logic to be thought of as if it lives in an isometric world.