This might be because these contacts do not have a phone number associated with their phonebook entry, e.g. email contacts.
There are many accounts synced with your device, some of them keep their own contact lists.
Some apps, mainly emails, have saved contacts who only have the email address info field populated, while the phone number field is left empty.
By default, drupe does not display contacts who do not have phone numbers associated with them, but if you prefer to keep them around, go to drupe settings -> Contacts -> Advanced -> check "Present contacts without phone numbers".
If you still find that some of your contacts don't appear in drupe, this may be because they are stored in an account that has not been perfectly synced with your device. Try redoing the synchronization by going to your device settings -> Accounts -> Select the required account and resync. Once your accounts are synced and the contacts associated with it appear in the device's phonebook, they should also appear in drupe.