Before syncing your tickets from your support system (Zendesk, Freshdesk etc) remember that you first have to have your accounts (customers) and all users in Planhat.
If users are added after syncing your tickets, only current and future tickets will show - i.e no historical tickets will show up in Planhat for users added after the sync is done.
If you have not added all users, and you do the ticketing sync....you should turn off the ticketing sync, add the users then turn on the ticketing sync again. Depending on the amount of historical tickets, it may take up to a few hours to bring everything in.
Here is the maximum amount of historical tickets that will be brought in (assuming users and companies are in Planhat)
Zendesk: All historical tickets
Desk.com: The last 35,000 tickets
Freshdesk: The last 15,000 tickets