Only in a general sense because we have no idea what is happening.
But what you do is place statements in the `zenphoto_sendmail()` function (and perhaps the `UTF8->send_mail()` function as well) to record what these functions are doing. You will see that they are really quite simple functions, So, for instance, you can dump out things like the parameters being passed to `UTF8->send_mail()` and be sure that it is being called for both the admin and the user.
See what paths are taken through each function as well. Generally you need to find some clue as to why one mail is not being delivered and the other is.