So I've recently watched Saving Private Ryan, The Longest Day, and A Walk in the Sun. They all seem to be very well researched and try to be as historically accurate as a fictional movie can be, but in all three of the movies soldiers (U.S. soldiers, anyway) don't buckle the chin-straps of their helmets very often, even when they know they are going into battle. There are even several scenes where a soldiers helmet comes off and he has to put it back on again. Were the chin-straps that annoying, or not very effective, or is this not historically accurate?
The chin straps of helmets were not typically fastened because, in some cases, their use posed a threat to the soldier's life.
If a soldier is wearing a helmet and has the chin strap fastened and properly located, any shot or concussive force impacting the helmet has the ability to snap the helmet back and potentially break a soldier's neck.
Chin straps could be worn in non-combat situations where a soldier ran the risk of losing their helmet, such as on a ship or navigating a demanding geographic feature, but commonly in combat situations the strap was undone or put behind the soldier's head to eliminate the risk of suffering whiplash or a broken neck from concussive force or direct impact.
The series Band of Brothers which i recommend you watch (its awesome and pretty accurate) has some of the paratroopers talking amongst themselves in the 1st and 2nd episodes. In the 1st a bullet that hits the side of the helmet could potentially spin it in a way that breaks the neck or at least chokes the soldier rendering him ineffective for a few minutes.
The 2nd episode deals with paratroopers that choked to death/snapped their necks when their parachutes got stuck on trees.
These conversations might shed some light on the issue.