I watched the Rookie, season 1 was 20 episodes, however season 2 was only 10 episodes. ABC waited until almost the last week to decide if to cancel and with only ordering 10 episodes, it sounds like they're not too committed to a season 3 unless the ratings picked up.
Personally I really like the show in season 1, but by season 2 I seemed to care less. A lot of new people was brought onboard. Nathan had a love interest, then they brought back an old flame from college, and suddenly he had two. The show started off season 1 where he was fooling around with another rookie cop. They cooled the affair because the superiors said that wasn't a good idea. I would like to see him move onto something else. On Castle he was a big shot author and wealthy, on the Rookie he's a lowly newbe cop in a westcoast city.