I used a Unit Reinforcement bonus but I did not get all the units.
More than likely you did not have enough free population. To create new units, even with a unit boost item, you need to have the required population. Or, in case of mythical creatures, you need to worship the proper god in your temple.
There is also a popup warning you when you don't have enough free population. If you're not seeing this, check in the settings under 'Confirmation prompts' to see if it is deactivated.