JAM has two different fields UserID and Username. Not sure which one is in effect here.
Where did you change this field? Must be in your SFSF? Then, it will not get reflected in your JAM immediately. Both SFSF and JAM are two different systems integrated with a nightly data sync!
If you got your JAM instance as a part of SFSF, then you will have to create users in your SFSF so that they will be replicated in your JAM instance in a nightly sync!
Make sure the new users are made Active and permission are set in such a way to allow JAM Access. Let's try with a new user to avoid confusions.
So, create one user in SFSF today and wait till it get reflected in your JAM instance. We will then have perfect scenario to test.
I hope you are from INDIA. Isn't it late for bed? 