A contacts is synched if their Klaviyo profile is linked to their charles profile. For this to happen, their Klaviyo needs to have the phone number used for WhatsApp on file or their charles profile needs to have the email address used for Klaviyo on file. The synched status is required to see the charles opt-in status in Klaviyo (using it for segmentation).
Here are the steps you can go to find out the current ratio of contacts we can also identify in Klaviyo:
Example:
Collect email addresses in charles via WhatsApp campaign
One starting point to increase this ratio could be to have a raffle or incentivization for getting e-mail addresses. This of course only should be sent to double opt-in contacts. If you are using Single Opt-Ins, you could go the following steps: