Your submission is in the 20-meter radius of a different POI (not all POIs are in Pokemon Go, but still count for the distance rule). If your submission isn't in ingress, it got sorted out after the approval by the Nia system. You can submit it again with a different location (only if you got a specific area, for example on a playground, do not fake the location because of this rule!)
Your submission is in a full lvl 17 s2 cell (only for Pokemon Go)
Your submission is newly accepted and you have to wait a bit until it is getting synced to Pokemon Go (up to 34 hours)
A syncing error is blocking newly submitted POIs to get into the games, so it can take a little longer than 34 hours until they will appear in ING/PGO. Mostly, only one game got syncing errors, so you could check the other games to check for your POI.
The location of the POI is in a geoblocked area, meaning that POIs inside this area might not spawn in a specific Niantic game. Also, it is possible, that some POIs might not qualify for a specific status ingame.
Answers
Q: My accepted submission is not ingame. Why?
A: There are five possibilities: