Why I can't train more Units? My Training Queue has not reached its limit.
The situation may occur when you activate any of the Dragon Arts that influence Unit Training Speed. The indicated maximum size of the Training Queue may be larger than how many Units you can actually train. This is because while Unit Training Speed has increased because of the Dragon Art, the amount of Units that can be trained at any one time has not.
For as long as such Dragon Art Bonuses are active, you can simply train the same amount of Units faster.
If you can normally train 100 Pikemen in 500 minutes, and then you activate the maximum 20% Bonus, you will be able to train the same 100 Pikemen in 400 minutes. However, you will not be able to train 125 Pikemen in 500 minutes, as the Training Queue will have shortened in accordance with the size of the training speed Bonus.