Bluetooth contacts come and go

Hello,
I'm new to this site as I recently purchased a 2011 Prius three with a solar roof.
So far I've found these forums to be a great source of information.
I have my iPhone 5SE paired with my JBL Navigation radio just fine. And I've synced a small group of contacts ('Prius Contacts') successfully.
But the contacts seem to come and go at will. I'll see them fine on the drive to work but they may be missing the next morning and back on the drive home.
I've un-paired and re-paired a few times and reset the network settings on the iPhone, but the problem remains.
Has anyone else seen an issue like this?
Any tips to get it working more consistently?
Cheers