Weapons groups can only be set on the 1- 4 keys. Keys 5 - 8 are target keys, i.e. in mission you select a target and press Ctrl + 5, 6 ,7 or 8 and it will target that unit for the rest of the mission just by pressing that button. It saves time when there are multiple targets and there are some you want to keep an eye on. If the 1 - 4 keys are not accepting the input (after setting the group you must press Ctrl + 1, 2, 3 or 4) I don't know what the problem is. I've never heard that one befroe.