My heat/air only blows out front vents. Will not blow out of defrost vent or floor vents. What might be causing this? It's a 2015 Ford Explorer.

SOURCE: Heat wont blow through heater or defroster vents

common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.

SOURCE: Vent control

You have a vacuum leak.Defrost is the default postion(no vacuum).
See if you can locate the small black vacuum line that goes inside the vehicle on the passenger side near the heater core. The trace it back,should see it going to a canister then to the intake manifold.Make sure it is not disconnected or broken somewhere.

SOURCE: PROBLEM WHEN SELECTING THE DIRECTION FOR THE AIR

Sounds like you have a supply vacuum problem. When you lose supply vacuum to the control head it defaults to defrost.

SOURCE: All Air Conditioning blows our the defrost vents

Some thing you could look at would be this. The control is usually fed by a vacuum line or small hose thats is connected to your intake manifold.
With a cold engine,look for a skinny vacuum hose that runs from the intake toward the fire wall . It should be going inside the cab and connecting to the A/C vent control.Some times the hose becomes disconnected or splits or cracks due to heat under the hood.
Thats usually the trouble.
I hope this helps you.

SOURCE: when i turn the air on it doesn't flow from the front vents.

Sounds like the switch or actuator is not operating. Some vehicles have mechanical operators, and some have vacuum actuators. If a vacuum line is loose, it will not actuate.