Why is my observation not showing up in a Place or Collection project? I know I observed it there.

The most common cause has to do with how iNaturalist associates observations with a Place. Each Place boundary has what is called a "bounding box," which is a rectangle of latitude/longitude lines that contains the entire boundary. 

For example, below in red is the approximate bounding box for Lake Merritt (in yellow), in Oakland, California, USA:

iNaturalist will not index an observation as being in Lake Merritt if either the observation's accuracy circle or obscuration rectangle have areas outside of that bounding box. 

We do this to prevent observations from being added to a place when there's a chance they were not found there and, more importantly, to prevent users from narrowing down the location of an obscured observation. This means that if you have a Collection Project for a small place, obscured observations as well as observations made near the edge of the boundary may not be displayed in your project and you may want to consider using a Traditional Project.

Note that this does not apply to counties, states, and countries or their equivalents, which are "Standard Places" in iNaturalist (as opposed to “Community Curated Places” that users can add).