Here in the USA, those Tracfone SIMs don't get detected so it's true that not all SIMs will work. Be sure to pop in your old working SIM to test again.
Just bough the unit August 2012 in Dubai it works fine in Vietnam SIM (Viettel). Now I'm in Turkmenistan I used 3G TM cell SIM but my unit doesn't detect SIM. Is it possible that Galaxy Tab2 10.1 GT5100 is not working with some SIM of network provider?