Why won't my nano SIM work in non-iphone handset?

Before you ask, YES, I have an adaptor so the nano SIM does physically fit in the non-iPhone handset (SGSIII and HTC 1X), the handset just won't register that there's a SIM card installed. It's strange because it has been working in a non-iPhone product (sending the iPhone 5 back as it's pants in case you're wondering why I have a nano SIM in the first place!) but now it's not working and I've tried in both the Samsung Galaxy SIII (it has worked in there without issue until today) and also the HTC One X but it will not work in either. The SIM card is on the Three network (don't ask - they're AWFUL!) so I'm wondering whether they've blocked the SIM from working in anything other than the iPhone 5? As explained already, the nano SIM HAS worked in my other handsets but stopped this morning. Any ideas/recommendations gratefully received - that is to say short of getting a micro-SIM in its place - I WILL be doing that at the weekend as I can't get to a Three store until then so just need a temporary work-around if there's one. Thanks for listening peeps!

Added (1). Kewko, the phone is UNLOCKED - never has been locked to any network so it's not that - and it WAS working this morning, just suddenly stopped. I've just done a full factory reset and have the same problem. The SIM card works in my other unlocked handset - it just seems to be the SGSIII that has the problem. Going to the Carphone Warehouse now to try and sort out.

Are you sure the phone you are trying to put it in is not locked for a specific network?