My breasts sometimes take a little time to get going, too. And often, if I pump for a full 15 minutes, I will have another let down, so I am sure that is what happened to you. I think if I pumped for an hour, I would have other letdowns, too, because breasts continue making milk the whole time. But who wants to sit there for an hour?
Don't worry about it taking a few minutes before your breasts get going. I think that is normal.