Detail your problem, where are you able to connect to the database from. Where is your phpMyAdmin located.
It would help you to know that user accounts in MySQL are made up of a username and a host that the username is allowed to connect from. So the account name is 'username@host'. An account john@mydomain may be able to connect to the MySQL server does not mean that the account john@domain2 would be able to connect to the same MySQL server too. This is what allows two persons with the same username but on a different machine to be identified as two different user accounts in MySQL and hence be controlled differently.
So when you say, you are able to connect to the server from, say, your PC does not mean that you should be able to connect from phpMyAdmin as well, cause, the phpMyAdmin module could be located on a different server.
MySQL user permissions can be very very confusing, for the reasons that verruckt24 mentioned. Just because you've set permissions for a user one way doesn't mean he/she will be able to connect another way.
Have you tried using the system root password to log in to phpMyAdmin? Is this server yours, or are you using a hosting service?