Is it an old miniSIM? There are two styles of miniSIM (aka 'normal' SIM which you can cut down to microSIM size). The old miniSIM is the same size as the new-style miniSIM, but the gold contact area is bigger, and the 'brain' of the SIM is also bigger, so when I cut it down to microSIM size I killed it.

I fixed it by asking my network for a new-style miniSIM and cutting that down to microSIM size.