Hi @Cal90,
It depends on which maps you are actually using. I use the downloadable maps which are for offline use, and are updated monthly. Because they are updated monthly, in an extreme case, it can be over a month before a postcode I have added to OSM is actually searchable in Osmand. In example, if I added the postcode AB12 3CD on the 2/1/2021, on the 1/2/2021 Osmand downloads the new OSM database and creates the new maps. For me, the new map will only be downloadable about the 12th Feb, so it will not be searchable for about 6 weeks from the date I added it.
If you're using the online tiles, I can't really help, as I've never done that.
Regards
Nick
OSM = Tallguy