The most common case for the shield flipping up is usually a result of the shield being removed or changed and then re-installed improperly. The way to verify this is to check to make certain that the shield tab is under the channel on the shield based plate, see our Helmet Tech section.
If the tab is in fact under the channel (on both sides) then there is a possibility that the actual base plate position needs to be adjusted. By loosening the base plate screws and then moving both base plates backward will increase the closed tension between the shield and the eyeport gasket increasing the strength of the shield in full closed position. Please be sure not to over tighten the base plate screws.