HERE+ RTK surveyin duration

Hello, I have been trying to use here+ rtk module with mission planner, but I get inconsistent survey in durations: sometimes I reach 2m accuracy with-in 10min sometimes it stuck around 7m after 1hour. Also sometimes accuracy increases instead of a steady decline.
Any suggestions?
PS: rtk firmware is 1.43. I do my tests roughly at the same spot: at the roof of a building.