[iMore Question]

It worked every time when not in cover, when inserted in cover, kept getting repeated Try Again then Passcode...
Took it out of case, worked like a charm.

Also wonder if the plastic cover on back of phone causing this problem?

[HankAZ]

It worked every time when not in cover, when inserted in cover, kept getting repeated Try Again then Passcode...
Took it out of case, worked like a charm.

Also wonder if the plastic cover on back of phone causing this problem?

The TouchID sensor is in the Home button, so anything you do to the back of the phone will not affect it. If you trained the TouchID out of a case, the issue may be that your are not aligning your finger on the Home button the same way you did when you trained TouchID. I also expect that the additional thickness of any case will cause you to alter the way youre holding the phone, relative to how you held it when it was not in a case.

Try adding another finger, but do the training with the phone IN the case. See if that makes a difference. I suspect that it will.