I work in a team of 20 and every time it is someones birthday we always sign a card and the manager usually buys a small gift from the team.
Last week was my birthday and it wasn't acknowledged, I didn't work but other staff who haven't worked on their birthdays always receive the card before.
The manager has a list of everyones birthdays so there's no way he didn't know.
I feel a bit upset and that maybe the team don't like me and didn't want to sign the card(I know it sounds pathetic).