You must ensure the account you have logged in with, matches the account you ordered your tickets through. The easiest way to tell is to look for your email receipt and check the email address it was sent to.
We also often find that customers have logged in via Facebook which may be linked to an old or unused email address.
For some high profile events, we may have suspended access to your barcodes to stop those pesky touts.
If your app login screen looks like this then please head here for further guidance.