[Lt. Ketch]

It depends on how you're trying to load them and if the transport has enough room.

The best way to load a unit, in my opinion, is simply to make sure the the unit you want to load and the transport on to which you want to load it are in the same hex then click on one of them, hit the "L" key and then click on the other. If there is already a unit in the transport and you wish to load an additional unit into it (assuming there is enough room for both) you need to click on the unit BEING loaded, then hit "L" and click on the transport. If you click on the transport and hit "L" it will unload the units currently sitting in/on it. This should work in most, if not all, situations.

If this isn't helpful, I know there are more knowledgeable and articulate people on the forum that can help with problems this didn't work for.

Welcome to the game and forum!