403 for some users in Development Mode

[sirglag1]

I am using Web API in Development Mode for prototyping an idea.  It runs on the web using React and Node.  I am not using any Spotify libraries, just hitting Web API directly.  

The endpoints I am using are:
-  /v1/me/playlists?limit=20
-  /v1/me/top/tracks?limit=15
-  /v1/playlists/${playlistId}/tracks?limit=100

Now when I use my app locally or on my deployed app it doesn't cause a problem (no 403s).  However for some people I am testing with (they live in Poland) they see 403.  It worked once for them, but now the endpoints consistently return 403.

Is there something I need to do?  Do I need to add them in my User Management for them to be able to test my app?  If so, why did it work for them before?

I am using typical oauth flow and using their access_token for these calls.

Thank you.

[LambertSpot]

Yes, they need to be added in the user management if your app is in development mode. 

[sirglag1]

Thank you so much for your reply.  Any idea why it worked once or twice before for them?  Was it just a fluke?