MIssing one of your fleet slots is usually caused by resetting the National Policy (Command Center)

When you reset your National Policy, this will also remove all the buffs you've gained from the policies you have selected.

To obtain the +1 deployed fleet unit, you need to unlock either of these policies. 

- Liberalism > Volunteer Army
- Despotism > Royal Bodyguards

To access the National Policy:

- Go to the Command Center facility

- Tap the Policy icon

- Tap 'I' (for Tier I

 More Tips and Hints... 

National Policy & The Importance of Resetting