a common issue with these iMac's is getting the CPU chip to properly sit in the socket and sometimes the socket pins get messed up a bit.
The spring is a bit too strong and the RAM cowling gets in the way! Even I have problems!
You'll need to go back in and inspect the pins be very careful as they are easy to snap. Reshape the springs a bit to get some of the tension off. Don't go to far as the pressure is needed to hold things tight.