As a general point - extremely bad practice to slap the arm in an attempt to raise the vein. Has the potential to compromise the quality of any blood sample that comes out.
Approved guidance is to ask patient to lightly clench fist. Should vein be difficult to visualise or feel, check other arm. No easy to visualise vein, apply localised warming for a few minutes.
Whether a vein will show or not is down to age, genetics, fat or lack of it, hydration level, warmth - all sorts really.