Problem with new door lock - can't open

[OM2]

I've fitted a new lock
It's an Avocet ATK 3 star

Today we shut the front door and double locked
When we came back and tried to open... the key wouldn't turn

I had to come in through the back garden

The door lock opens from inside fine. With the door open, it can be double locked and unlocked many times with no problem

Only when the door is shut and I double lock is there a problem. If I turn the key and keep the key in, then turning back to open is OK

Only when I take the key out... there is a problem 50% of the time

What should I do?
Someone suggested adding WD40?

Thanks

 

[foxhole]

Have you secured with locking screw? May be slightly misaligned causing snib to catch when unlocking . Try removing screw and test lock without it .

 

[OM2]

I've taken out screw, without the screw it's fine

I've tried using the old screw from the previous lock which is Shorter. Same problem

Not sure how I can misalign
Where to confuse what to do now

 

[foxhole]

I've taken out screw, without the screw it's fine

I've tried using the old screw from the previous lock which is Shorter. Same problem

Not sure how I can misalign
Where to confuse what to do now

The lock can bind if not dead centre , even a few mm out is enough to cause problems .
Don’t over tighten screw .

 

[OM2]

The lock can bind if not dead centre , even a few mm out is enough to cause problems .
Don’t over tighten screw .

I've actually left really loose. Still the same
I can give it a go pushing back or forward - but don't think that will do anything
I already tried last night, same result

'graphite lock lube' - I saw someone on youtube use this (really old video)
Will that help me? (I think it would be ridiculous to buy something extra to be honest!)