Juce apps not showing up on some older devices

I was testing my app with an older Nexus 5 today and found that with API 23 the app never loads and the only thing I get is an empty application frame with the titlebar.

I did a little digging with breakpoints in the cpp code and it seems it never initialises the the Juce app.

Android studio gives the following error:
Cannot resolve corresponding JNI function Java_com_rmsl_juce_Java_initialiseJUCE. Could not find an existing file with JNI definitions in it. Please add a file with JNI definitions to the project in order to get suggestions.

Update: now the error went away after saving and opening from Projucer. Still no content on an app that works perfectly on iOS, Windows and Mac.

Any tips on what I should try to make it work?

Thanks.