Could be many different things and depend on the corner. First question to ask yourself would be “Is this always on the same corner I have trouble?” and then “Do I get it right some times?” Try to get some visual cue on where you brake when it work well and same goes for your turn in. Your turn in should be the same all the time.
As you get better your speed also increase coming out of the previous corner and will change the work you need to do in that next braking zone. Usually the increment in speed is quite large from session to session when you begin so pay attention to your corner exit speed. Adjust braking based on that. Stay smooth on and off the break (press hard but don’t upset the car balance).