Dual SIM adapter issue - iPhone 5?

The dual adapter works fine when I have a payg Orange and a payg T-Mobile SIM in place, but whenever I remove either SIM and insert my contract EE SIM, the phone displays 'invalid SIM'.
My EE SIM works when used in the phone without the adapter.
I'm obviously wanting to use my contract EE SIM in conjunction with any of the 2 payg SIMS.

Any ideas are greatly appreciated.

Sounds like your iPhone 5 is NOT unlocked, so it will be limited to certain SIMs it will work with.