You could try sitting with your heel elevated, eg. on a chair of the same height and just leave the leg to straighten out passively by sitting there like that for 30 mins, or as long as you can bear. Just do this as often as possible. I've sat like that for an hour a time around 6x a day.
Do check with your physio that she is happy for you to do that.
PS It will hurt so you may have to reduce the duration but your knee should straighten out unless there is something mechanical to obstruct that. At least your physio is vigilant about this!