Comment 2 for bug 1724146

Thats an old problem discussed in different topics. In short:

To get a trainer you need a ware. Eg. a barbarian trainer needs an ax.
A Trainer is made only in a warehouse, if there is also the needed ware for the trainer in a warehouse.
As soon you build buildings which consumes axes, like barracks, all axes are transported to those buildings and no one is left to build a trainer.

If i ran out of trainers i set the stock for ax in barracks to zero. That way produced axes will go into a warehouse and a trainer could be build.

I set this bug as duplicate of bug 1422158.