知らない　 always 'I don't know' わかりません　could be 'I don't know', could be 'I don't understand'. It depends. e.g.) When is his birthday? ----> you can say '知らない' or 'わかりません' both are ok. But if you don't understand when a Japanese person talks to you, you should say 'わかりません'. In this case, '知りません' doesn't work.