memory works in slot 1, not in slot 0

I tried upgrading my iMac (7,1) (2 ghz intel core 2 duo) to 4 gb ram using 2 gb sodimm.

After putting the 2 sodimm in, the computer turns on (HD spins) but nothing happens.

Replaced the 2 gb sodimm in slot 0 with a 1 gb that had been in there and the iMac chimes right on.

Replaced the 2 gb sodimm in slot 1 with the 2 gb sodimm that had been in slot 0 and it chimes on.

I put the 2 gb sodimm that had been in slot 1 in slot 0 and, as before, the HD spins, but no sound & nothing on the screen

(as the first attempt)

Put the 1 gb sodimm in slot 0 and everything works again.

About this Mac reports 1 gb in slot 0 and 2 gb in slot 1.

I waited about one minute each time I had a 2 gb in slot 0 with only the sound of the drive spinning.

With 1 gb in slot 0, the chime comes shortly after power on as normal.

Everything that I have read says I should be able to go to 4 gb.

Anyone have any ideas on what is going on?

Thanks in advance for any advice.