[Q] Thl 4000 dual sim problem topic

Hi, have thl 4000 - love it! However I can't use two sim cards in it. For some reason one card is working perfectly in ether slot however the other (which works fine in iPhone 4s) is not working in nether slot. Moreover if I keep working card in one slot and put not working one in the other nether sim card is seen :(
This is my first dual sim card so maybe I'm doing something wrong? Would appreciate any advice.
Romas