Please Help!

My girlfriend and I are leaving on Wednesday from Hanover to Nizza via Mannheim - Paris Est. Thats an ICE and we have to have seat reservations. Unfortunatly it’s just not possible to reserve any seating on this connection. Not via the Interrail Website, nor via Deutsche Bahn. Why is that and how can i fix it?? Do i have to make a reservation by calling somewhere or what do i have to do? 

Thanks for your help,

Tom and Luisa