There can be any number of reasons why your manager did not send you a birthday message, but if you start trying to read other messages into this, you are not going to have a happy birthday at all. Your manager may have been busy, preoccupied, or uniformed about your birthday, or perhaps he just forgot or planned to send something at the end of the day, but it got away from him.
If you go to your manager and ask him why he did not send you a birthday note, you can easily sound insecure, dependent, and petty. Besides, if you have to remind him to send a card, how meaningful will it be? You should let this matter go. If you are looking for birthday wishes from him in the future, try dropping a few hints.
The best way to determine if your manager is trying to send you as message is to look at the formal and informal feedback that he provides on your performance. Besides, there are some employees who get perfunctory birthday notes from managers, but such notes can be the only positive feedback they receive.
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