[Pointless]

I'm having some issues with the central locking. All the car doors will
Lock and uplock fine but I have to lock the barn
Door with the key...

Anyone had this before and know how to fix
It?

 

[ReefMagnet]

Possible causes
1) Solenoid broke
2) Wire to solenoid broke
3) Stiff lock mechanicals

Possible fixes
1) replace solenoid with aftermarket or oem
2) Find break in wire and fix
3) Clean and lubricate or replace

 

[A4B]

As above, and also check the doors are closing/adjusted correctly.

Also the black sealer that retains the plastic water shield sheeting (behind door card) can sometimes run down and restrict lock rod movements.