There are a few reasons why this may be happening.
Your contacts are stored on your phone's SIM card.
Due to the severe limitations of storing contacts on SIM cards, Covve doesn't sync SIM contacts. It is generally advisable and good practice to move your contacts to a "cloud" storage account so that they're also backed up in case you lose your phone.
Your contacts are stored in a locked Exchange account (iPhone only).
Some corporate Exchange servers do not allow third party applications to access your contacts. If this is the case you'll either need to request a policy change from your Exchange administrator or copy the contacts to another account
Your contacts are stored group/account which Covve doesn't yet support (Android only).
Covve syncs with all major address book groups/accounts such as Gmail and Yahoo. If your contacts are stored in another account which Covve doesn't sync with just let us know at firstname.lastname@example.org and we'll make sure to support it in the near future!