The other thing you can sometimes try (if you don't have a corps) is Selecting the other "unit" first. If it's a depot, pick the garrison instead. If you had the garrison selected, pick the depot or corps. That kind of thing.
What I've found most about this is that it is highly inconsistent. If you change focus to another city and then come back, sometimes you can do what was not happening just a few clicks ago.
Another thing to watch for the is the presence of other factors in the city. If, for example, another power at war with this nations owner has 7 factors in the city, then it would be considered full.
Finally, be careful that your own corps counter is not in the city. You can't transfer factors from a garrison unless the corps is outside the city. But, for some reason, this doesn't always get enforced, oddly changing behavior in the middle of a transfer.
At LAST! The greatest campaign board game of all time is finally available for the PC. Can my old heart stand the strain?