The three things people actually write in about.
Almost always one setting, and it is not the one you would expect. Android treats daily reminders as alarms, and that needs its own permission — separate from the notification permission you already granted. Without it the app schedules reminders and Android quietly drops every one.
Turn it on, then open LinguaNews once so it can reschedule. This alone fixes most cases.
Some phones close background apps aggressively to save battery, which can swallow a reminder even when it is scheduled correctly. The wording differs by manufacturer:
To check it worked without waiting a whole day: open LinguaNews, go to Settings, tap the reminder time and set it a few minutes ahead. Then close the app completely and wait. If it arrives, you are set — put your usual time back afterwards.
Your progress lives on your device and, when you are signed in with Google, a copy is kept in the cloud. Uninstalling clears the device copy. If you were signed in, sign in again with the same Google account and everything comes back on the next launch.
If you were never signed in, there was no cloud copy to restore from. Signing in now protects everything from here on.
A perfect day needs both halves: a lesson and a vocabulary review, on the same day. Either one on its own keeps your streak alive, which is why a day can hold your streak without turning gold.
Seven perfect days in one Monday-to-Sunday week make a perfect week. If you miss a day, that week can no longer be perfect on merit — so one missed day a week can be bought back with diamonds, from the ring on the home screen.
Write to linguanewsapp@gmail.com and say which phone you have. Reminder problems are nearly always manufacturer-specific, and knowing the model is usually enough to name the setting.