If you go to make a reservation and the system is saying you have already maxed out your reservations please check the rules that are listed on the reservation page. Some locations will have rules such as an account and/or plate can have a maximum of X amount of reservations at the one location.
This will prevent you from making a new reservation until one of them expires or is cancelled.
The rules are always listed in the FAQ of the Parking Website and on the reservation website as well.