heater in my 2006 Mazda

I have a 2006 Mazda. Sometimes, when I start the car, the heater will not turn on. If I drive it 10 or 15 minutes, then the heater will kick in and begin to work. The problem started a week ago and does not occur all the time. At first, it occurred when the car sat for 5 or 10 minutes then I restarted the car. Now, it has happened twice since randomly then but the 5 or 10 minute stop and start does not occur.