Each increase in rank requires higher levels of on the boat experience.
Crew Rank only requires crew experience
Skipper Small Boat Rank (25 feet and smaller) experience requires small boat experience
Skipper Rank (large boat (25 ft and greater) requires sufficient experience on 25 ft (7.5 m) or greater
Bareboat Charter Master Rank requires experience sufficient on a 28 ft (8.5 m) boat
Captain Rank is the same as Bareboat Charter Master
If your level went down - it is because you started out on Skipper Small Keelboat and you reached levels because you have experience on smaller boats. Once you jump up to Skipper your logbook requirement is that you have some of the experience on larger boats.
If you level is staying the same despite entering time into your logbook it is becasue then boat size or type is not matching what is required for the Rank. e.g. if you want to rise in level when you are skipper you must enter more time on a vessel greater than 25 ft.
For complete information see https://www.nauticed.org/faq-sailing-certificate#sailinglevels