An educated guess says that you are using a specific media query for devices, rather than resolution alone, which is why it's not responding when you resize the window.
Mobile devices: (what i suspect you have)
@media only screen
and (min-device-width : 320px)
and (max-device-width : 480px) {
/* Styles */
}
Based on Resolution: (what you require)
@media only screen
and (min-width : 320px) {
/* Styles */
}