Why can't I train more Units when it shows my Unit Training Queue isn't full yet?
This can happen when you activate any of the Dragon Arts that affect Unit Training Speed. The indicated maximum size of the Training Queue may look larger than the number of Units you can actually train, but it isn't: 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.
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 along with the training time.
This is important since the Dragon Art Bonus might only be active for 180 of those minutes. Once the Dragon Art wears off, your queues will return to their original size.