The problem could be with your ID card or with the specific door access. If, when you attempt to get into an entryway, the card reader flashes green but then quickly turns red, you have not been granted access to this entryway. You will need to contact the business manager of the building you are trying to access.
If, when you attempt to get into an entryway, the reader does not light up at all, this would indicate there is likely something wrong with the ID card. Please bring the card to the ID Center for further troubleshooting and replacement. Note that as long as you bring your current card to the ID Center, you will not be charged for a replacement in the event the card is no longer functioning.