There's a lot of information on that at https://www.justis.nl/producten/vog/beoordeling-besluit-en-b..., too much to summarise here, but I'll just mention what it says at the end: if you aren't given your VOG when you believe you should be, it's possible to ask for that decision to be reviewed. If upon review you still aren't given your VOG, you can appeal that decision by taking it to court and having a judge look at it.