With a PD(similar to MZ) most people report 3-5 nice hits out of one stem.
You need the right light and backround surface to really SEE vapor, so I cant say whether you are.
If your taking hits, not holding them in, and then exhaling quickly in front of a dark colored, well lit surface, and dont see anything, its safe to say its done.
Edit: and usually you should hold vapor in your lungs for at least 10 seconds, FYI. you probably just have to refine your drawing technique, as well as grinding and packing to get good vapor. but you may already be doing that but you didnt say how many good hits you got before you stopped seeing vapor.