GLC 250D Phone System doesn't mirror contacts

[wirralpaul]

This is an odd one. My phone is an iphone 10 and is paired and synced with the vehicle. But, when I try to make a call it's hit and miss as to what contacts it finds. Sometimes it finds a contact and sometimes it doesn't. The problem is that the contacts appear to be intermittent. Today someone will be on the list and tomorrow they won't. The the day after they're back on it.

Any ideas ?