When you're in prone position, you can't use the prone key to stand up again. Need to use the crouch key to go crouched and then again to stand up.
Looks like the default key is assigned to "Prone" instead of the "Prone or stand". Minor oversight in the default controls.
On keyboard press Z to go Prone. Press Z again and nothing happens. Need to press X to go crouched and X again to stand up. Or need to double tap X.
Not sure about controllers. Might have the same kind of an issue.
Make it so the default control is "Prone or stand" instead of the "Prone"