There's three possible explanation to why your information (weights/reps) is not being saved:
It is being saved - you just can't find it.
Your information was never erased. It is all recorded in the Calendar section of the app. When doing an exercise, unless it is the exact same sets and same reps as a previous exercise you won’t see your past information. So when the sets and reps are the same as a past workout you will see the your data when you're doing an exercise. You always have access to your past information if you want to go back and check it. You can access it through the Calendar.
You aren't connected to the Internet when using Fitplan.
In order to properly use Fitplan you must be connected to the internet (wifi/cell data). You can pre-download the videos in order to save your cell data or avoid slow wifi in certain places, such as the gym. However, you must still be connected to the internet in order to save your progress.
When finishing your workout you didn't press HOLD TO END WORKOUT
At the end of every workout you must press HOLD TO END WORKOUT button in order for your progress to be saved.
If you are still facing issues regarding your information and your account please contact the Fitplan Team at firstname.lastname@example.org