Here is a picture of the inside of one of my WW2 M-1 helmets. They will adjust pretty small. I am thinking maybe you are missing a strap or band. Make sure you have the sweat band (tan colored band that runs the inner circumference of the helmet) This band is adjustable. Also make sure that you have the Nape Strap installed. This is the short 4"-5" strap that snaps into the back webbing and will sturdy up the helmet on your head..
Here is a pic explaining it all.......
Hope it helps
Michael Marietta GA