I had this problem...I had admin rights, I created my own site, yet I couldn't change the pwd for any of the users on my site.
this is what I found out.
- Site settings
- System Admin
- view site collection user information
- select the individual you wish to give the rights to change someone else's pwd (typically yourself)
- edit user information
then check the box: Site collection administrator
for some reason, after this....I was able to change other's pwd.