Problems with the new Apple iPhone 5S?

Christmas is coming up in a few days, and I'm excited to be receiving my iPhone 5S. I have heard mostly good things about it, but my mum has said that she has been experiencing some problems with the Touch ID. She says that 2 - 3 days after saving her fingerprint, it stops working.

For example:
Mum saves her thumb print on lets say, Monday.
She uses her thumb print to unlock her phone from then on.
On Wednesday - Thursday, her thumb print shows up unrecognised and doesn't unlock her phone.

Is this just a problem with her phone? Is anyone else experiencing these difficulties? Thanks in advanced.

Mums phone is an iPhone 5S Gold 16GB and mine will be an iPhone 5S Space Grey 32GB

Added (1). Both phones have been purchased from the official Apple website

She should use the same thumb and save it 4 times or so, don't use different fingers for each when saving, use the same thumb 4 times when you set it up.