From: Boston, MA
If you haven't played any titles that use John Tiller's general gaming system, then you will likely need to read the manual from cover to cover. Once you have played one of his games, many of the functions tend to carry over and then it is the ideosyncrasies that you need to learn. However the first time you play one of his games, you really need to read the manual from cover to cover in order to understand how to play it. It definitely isn't the most intuitive gaming system and feels quite long-in-the-tooth compared with many computer games these days. But the strategy and game-play is rock solid, so it is well worth the time investment.
This might not be the best choice of games if you prefer a "shooter" type game, since this will feel quite cumbersome. But it is an excellent game if you enjoy the cerebral/strategy side of gaming. For instance, units control the hexes immediately in front of them, thereby limiting enemy movement in adjacent hexes. This has the effect of preventing enemy units from flanking you if you are able to maintain a row of units together. However, you can get flanked if the enemy is able to break through your line of units. A lot of thought needs to go into how you arrange your units to "hold the line" and how the enemy goes about breaking a hole through the line.
But back to your original question, my guess is that you are attempting to move "fixed" units. According to the manual: "When an “F” on a red background appears [in the lower-center part of a unit’s data area that appears in the lower part of the screen], the unit is Fixed, meaning that it cannot leave its hex. A unit’s Fixed status is removed whenever its hex is attacked by enemy units, the unit becomes Routed or is “Released” according to the scenario’s Release Schedule."
Hope that helps,