Rome with icom card

Hi, is it just me or is it very unclear how to enter a museum in Rome when you fall under the 'free entrance ' category? Not one website from any museum has the option to reserve a free ticket to ensure your entrance. I have a ICOM card which grants me free entrance to many museums, but so far i had to get in line with everyone else because i had no reservation. Which is not possible to make... the websites are so incredibly unclear and when selecting tickets there is no free option.. Or am I really blind?? Where I am from you can reserve a ticket in every museum's website for 0€ if you have the ICOM card etc and ensure your entrance. So frustrating.