My iphone 6 and my ipad have the same contact list but some of the contacts on my ipad will not send a message. Also, these contacts will not sync when I send messages on my iphone. Any suggestions on how I can fix this?

Ensure that both your iPad and iPhone are logged-in to iCloud with the same AppleID - and ensure Text Message Forwarding is enabled on your iPhone:

Settings > Messages > Test Message Forwarding - set to ON - and explicitly enable your iPad from the device list.

More information here:

https://support.apple.com/en-us/HT208386

Hi,

Unfortunately, only iMessages, (the messages with a blue bubble) can sync across iCloud. With this information I am assuming that the messages that you cant see on your iPad are text messages or SMS (messages in green bubbles). This means that not all of your messages can be synced.

Sorry