I bought the Premium version, but the program shows me the free version.

I bought the application, but the app is still in Free version

Sometimes it takes a few minutes in order to get purchased approval from Google server (device should have Internet connection at that time)

I have bought Premium version, but suddenly the app switched to free version

  1. Find the app in “task manager” and close it
  2. Run the app again (if didn’t help, restart the phone)

I have several accounts and I have bought Premium version, but the app suddenly switched to free version

That is a well known bug in Android. You can try “Clear Data” and “Clear Cache” for Google Play Store.
  1. Go to the Android settings ⇾ Apps & notifications ⇾ Apps ⇾ find “Google Play Store” ⇾ Storage
  2. “Clear data” and “Clear cache”
More detailed instruction you can find by this link 

Also, you can read about another solution by this link