logic board wont work in new case.

I bought an broken iPhone 3G 8gb that turned on and worked but had a broken digitizer,lcd and rear case, i fixed it all and put it back together and it wouldnt turn on/connect to ituned. i also bought a 16gb 3g that had only a broken digitizer which i then fixed, so i switched the logic boards around and both of them worked fine but the 16gb is in the 8gb rear case. when i swap them back around again so that 8gb logic board is in the 8gb case it still wont turn on. the 16gb works in the 16 gb rear case fine. please help!!!!