No alarms for events in calendar despite reminders are made. What to do?

Hi Despite I make reminders (I get the bell symbol for the event) for events in calendar, I don't receive alarms any more. If I have Thunderbird open at the time for the alarm, I might get an alarm. If I then postpone the event I don't receive the new alarm. I have had Thunderbird and calendar for several years and it has worked without any problems, I would receive alarms occured for the time I didn't have Thunderbird open, when I opened Thunderbird. I did get the alarms, also when I postponed an event. But for long it hasn't worked. I can't rely on the calendar not receiving alarms/reminders. I have tried to delete/remove Thunderbird and install again, that didn't help. I hope someone can help me, please. Answers in Danish are welcome. Best Joyanna