How do reinforcements work?

[Zangorth]

I bought this game a couple of days ago, and I'm trying to figure out how reinforcements work. Everything I found online so far seems to indicate that as long as they're in the battle range, they'll automatically reinforce. And, in the beginning of the game, that did seem to be the case.

However, I'm at turn 290 now, and for the last many turns now one has done any reinforcing at all. I've tried several different things. I heard that maybe it doesn't work when you attack, only when you defend, so I tried defending, no reinforcements. I heard maybe there was a distance thing, so I stuck to armies on adjacent tiles and let them be attacked, no reinforcements. Most recently I tried building a guardian, whose primary purpose seems to be reinforcing, and again nothing. My city was just attacked, the guardian is sitting right next to the city, and apparently he just wants to watch.

Is this just a bug, or did is there a tech that prevents reinforcing, or what are the rules on this thing? Because it seemed to be work fine the first hundred turns or so, but now it's just not doing anything.

[Zentgraf]

Reinforcing works for both offense and defense. So if the armies are near each other, it should work. Some issues that I can think of:

1) Does the reinforcing army still have it's action point?
2) On the pre-battle screen, you can determine if armies will reinforce and in what order. There is a check box that needs to be checked if you want them to reinforce.
3) During battle, are the deployment tiles clear? The deployment tiles have your flag on them. If any unit occupies them, they are blocking reinforments from entering the battle.
4) Is the winter effect that blocks reinforcing currently in play?

[Groo the one]

In addition to what BZentgraf did write:

1) You can check out the upcoming battlefield and this way all armies beeing able to join battle. Choose the army to attack with and hold right mouse button, when hovering over targeted enemy army

2) I don't remember exactly, but when a city is sieged reinforcement mechnics can diifer. I.e. I'm not sure about, if other armies join battle in case of:
a) You raise an army from sieged city to attack sieging army
b) You attack a sieging army with an army from outside the city

3) In general keep in mind, some heroes can raise the number of reinforcement flags (from 2 to 4, if I'm not wrong) by skill, for to have more units joining as reinforcements each combat round - as long as these flags aren't blocked by enemy or friendly units

4) Related to your issue, I guess point 1) BZentgraf wrotes about could be the reason for

[Nemo, Forevermore]

Also, there is a random winter effect that prohibits reinforcement during winter. This may be your culprit.