If a student is having problems logging in, then please check the username and create for them a new password. To do this:
Login to the Teacher Dashboard and head to Manage > Students
Select your class from the drop down menu
Here you will be able to view your student's username and reset for them a password. To reset the password click on the pencil icon under the edit column
You'll then be presented with an 'Edit Student' box and will here be able to update the password
They should then be able to login to the Doodle apps successfully
The other thing to check is that the students are logging in through the correct student login method. When they open up their app, make sure they click on Login and then on the next page they see they select Student username and enter the username and password you have confirmed with them (remember, these are case sensitive!).
If they are seeing a different screen, they can press Back in the top left hand corner to navigate to the correct login page.