Hi there are some problems with interrail.eu reservation service. Which trains are you booking? You could use other website to do the same.

These are the seat reservations i want to book. 

Thalys reservations Köln-Paris should be possible at DB Reisezentrum, but I know that sometimes there are problems with their systems. Or online only with paper pass cover number via B-Europe or via SNCB Call Center. 

Barcelona-Valence is only possible via postal mail (except SNCF call center), when you buy it on Interrail.eu you get it via postal mail. You can buy this reservation at DB Reisezentrum. 

Chambery-Torino is only possible via SNCF Call center (press #85 for english, but they are not very happy about Interrailers...) or SNCB Call center or at Ticket Offices of SNCF or SNCB. You could also try to go via Nice and Ventimiglia with TER that do not need reservations and in Italy with Regionale Veloce or IC. 

For Italy you can use tickets.oebb.at use one way ticket option and then at 1x adult add the discount code Interrail/Eurail Global Pass. You can buy the reservation also at DB Reisezentrum. 10€ for Freccia and SBB-TI EC within Italy and IC 3€. 

For the Eurocity to Rosenheim you have only optional reservation like in Germany. You have to buy a for the italian part a fee of 10€ in 2nd class. You can buy online via tickets.oebb.at use one way ticket option and then at 1x adult add the discount code Interrail/Eurail Global Pass also at DB Reisezentrum, where you get for free a reservation for this train. 

Well, let's say experiences are mixed with the SNCF hotline. But in principle they can book anything with SNCF involvement.