... You need to leave the door open a little. If it's left closed during the shower, you create a vacume and the air doesn't move out of the room that well. Also, if it's not high enough on the wall, it won't help. And lastly, assuming all the aforementioned is 'addressed', perhaps the outside part of the vent isn't opening. If you can get up to the vent from the outside of the house while it's running, check to FEEL that it's venting air to the outside. (Obviously, you should feel the air.)
Even if you take care of all this, you can still have moisture in the bathroom. Do leave the fan on for a few minutes after you're finished. If it's turned off too soon, then moisture is left in the room. Just as an example, my wife's bathroom is approx. 6 x 8'. It has no vent and the window is opened about 1" or so (year round). By leaving the window open a while and by leaving the door open a little during the shower (and wide open when she's 'decent',), we have no mildew problem. Soooo, venting a bathroom w/o a vent (?) can be done w/o a vent. You should be able to do a better job with a vent.
If you need more info, post up. My best to ya and hope this helps.