Reaching maximum member capacity: if, for example, your fleet has been set to allow a maximum of 8 members, then 8 members must join the fleet for it to be considered at full capacity. After this happens, a 6-hour countdown will begin. However, if any members leave the fleet during these 6 hours, you will need to wait for the fleet to again reach maximum member capacity for the 6-hour countdown to begin again.
If sending a message to your fleet captain doesn't help and you don't wish to keep waiting, your only other option is to leave the fleet. We'd like to remind you that after you leave the fleet, any coins or food previously donated to the fleet will be lost! Next time you can also consider creating your own fleet to avoid a similar situation from happening again!