Hi fiatnm,
Yesterday we discussed the adaptive settings, and why we might turn those off.
As mentioned this is my preferred method, as it does not create adaptivity unless I choose to (by using the CTRL key when project geometry), but it does require us to constrain the parts, where as using adaptivity would not require constraints.
So if you are watching Youtube videos and have the adaptive options turned off, it's likely you will need to do some extra constraining that the video authors do not do.
I think the others are correct in that your assembly didn't get saved and that's why the part was missing, but the constraints might be due to the settings, if you turned them off.
Here are two quick videos.
In the first I leave the option in the Create New Part dialog box on, so one constraint (a Flush) is created automatically. Notice that my new part is not adaptive, and not constrained in place, so I add an insert constraint. The Flush constraint becomes extra, and could delete if I wanted.
In this second video I remove the option to create the Flush constraint automatically as I create the part. I still add the Insert constraint though.
I would need to save the assembly at the end of these videos, in order for the new part files to be created on my computer (it only exists in Inventor's memory, until I save it), and I need to save the assembly in order for the assembly file to record the link to the new file also.
I hope this helps.
Best of luck to you in all of your Inventor pursuits,
Curtis
http://inventortrenches.blogspot.com