Had you tried it before buying it, and did it last then? And are you sure you're not just getting olfactory fatigue--that is, if you spray like you're obsessed and then go to the office, can other people smell it?
I just ask because a woman who used to work next to me always thought her perfume had vanished, but every morning we could smell her before we could see her as she arrived. She'd then reapply several times throughout the day. Not so cool.