adding rooms to an already generated timetable

[chrischi]

did you also tried it with 0% instead of 95%?

Do you mean to change all the space constraints that are set to 95 % to 0 %?

[Liviu Lalescu]

but in the other case one of the preferred rooms is still empty, and I have no idea why. What could I do?

This is a minor problem of FET. FET does not care always if a room is available and an activity does not have a room, even if simply putting this activity in this room should solve the soft conflict. You need to manually solve this or strengthen the space constraints.