Try swapping the to the other slots & don’t forget to press really hard until you hear the ‘click’.
I found it best to make use of the factory ‘pull tabs’. Much much easier than trying to pull the ram out by hand. It may seem like a good idea but, don’t be tempted to use pliers.
Try your best not touch it in the first place
Press really hard, to the point you think you’ll cut your thumb, you’ll hear a slight ‘click’. If it’s not properly installed, the iMac will ‘beep, beep, beep’ In protest upon booting…