nvt reax/c

I am a student from china.I am annealing α-quartz to get amorphous silica using NVT ensemble. The potential function is reax/c.In simulating ,the Volume fluctuations is grate but the pressure almost unchanged. Why is this? Can you give some advice?
Best regards,
Liu qingsong

If you are using NVT, why is the volume fluctuating?