I did the same thing; couldn't get it to start up because I was being too gentle. You really have to force it in there, which I never would have expected. Oh well.
At first I thought I'd broken it, cause all I got was the flashing light on the front when I powered up.
I was really surprised with the amount of force you had to give it.
Here I thought I was the only one who had that problem. It really gave me a scare. They should tell you in the instructions that if your thumbs don't hurt then you aren't pushing hard enough.
Forcing a ram chip into an iMac logic board: nothing like leaning your body weight into an expensive piece of electronics waiting to hear a "good" click as opposed to a "bad" click! 😱
When it does happen though, you realise its a good click.
But the manual makes it seem like its SO easy.
I'll admit putting the sticks that came OUT of my iMac into my dad's macbook was a helluva lot easier than putting sticks into the iMac.