There are some basic minimum requirements (as always) for 10 to be offered up in the free giveaway. If your device doesn't meet those specs you won't get the little icon inviting you to "the party".
Here is what MS says
Windows 10 Specifications ? Microsoft
And here is a bit of a Q & A
Windows 10 FAQ & Tips ? Microsoft
I had the icon pop up on my desktop unit, but it wasn't showing up on my laptop, which as far as I could see met the specs as per what it says in that above link. It wasn't until I had downloaded an insider preview image and started loading it on the laptop that it then came up and said it couldn't continue because the processor didn't have NX capability, which wasn't stated anywhere obviously in the required specs.
So if there is something that doesn't meet specs, you won't get the invite to the party.