I'm having really difficulty adjusting to the side mounted finger print reader, this might be down to me not being used to it, I've registered 4 different prints and it seems very hit and miss.
On my Sony phones with side finger print readers, I found it worked best to register the same finger that you want to use for unlocking multiple times, as though they were different fingers. Try that.
The reader may also be unreliable if your finger is is wet or sweaty.