There are three possible answers for why Parkmobile's system will not allow you to extend a parking session.
1. The zone is not open for parking during the block of time you selected.
2. You have exceeded the amount of time you are allowed to park in this zone.
3. Your active parking session is for an area in which you are able to stop the parking session and the session has reached the maximum amount of time per parking transaction; however, if the zone's maximum parking time will allow you to stay parked for a longer period of time, you must stop the transaction and start a new transaction. Note that this means you will be initiating a new parking session which carries an additional transaction fee.