Also, be sure that your shoulder belts are close enough (as viewed from above).
Sometimes, in older cars that were raced before HANS usage was common, the attachment points for the belts are mounted too far apart.
About 10 years ago I was in an endurance race at Lime Rock when a driver wearing a HANS device (they were quite new then) struck another car. His belts slipped off his shoulders and his head hit the steering wheel. He did not survive. Under local law, police did an investigation and found that part of the problem was that the shoulder belts were mounted too far apart from each other. The HANS device (which was the old style without the ridges on the shoulder blades), may have actually "helped" the belts slip off. A terrible tragedy.
Here's some good info on this from motorsport-tools .com
"The shoulder strap mounting points should be lower than the point at which the straps bear down on the HANS, such that the straps make an angle of 10 Degrees to the horizontal. The mounting points must be equally disposed about the user’s centre line so that straps spread outward slightly from the mountings and around the HANS. This helps to keep the straps firmly engaged with the HANS."
If the mounting points for the belts are far behind the seat, the belt mounts should be very close together. (Belts may touch or even be crossed over each other if necessary) 75-100mm distance between inner edges (shown) is typical.
If the mounting points are close to the seat, the width between the inner edges of the belts should be equal to or 25mm (1 inch) less than the width of the HANS Device
HANS works with any 2" or 3" shoulder harness.
Mount shoulder belts of harness 25-50mm below the horizontal. ( MSA quote "10 Degrees to the horizontal"