Remember that if you turn Messages on in iCloud settings, only messages from that point in time get saved to the Cloud. So, older messages previously received/sent on that particular do not sync with iCloud.
So, based on that. Assuming that the iPad has always had that setting turned on, then all of those messages should be in iCloud. If however, you turned it on later, then there will be messages found on the iPad that are not in iCloud.
Now, I assume you have that setting turned on for the Mac as well. If not, turn on that setting and the missing messages may sync from iCloud. If the setting is already turned on, then it would seem that the iPad was not syncing with iCloud from the get-go.
If you can't get the messages via iCloud, then you might want to consider using a program like iExplorer or iMazing. Install either program on your Mac and connect your iPad via the charging cable. You will be able to copy the missing messages found on the iPad. Bear in mind. you may need to pay money for full functionality of the software. I personally have used both programs and both work well and reasonably fair on pricing.
Hope that helps !!!