New lock and plate... can't get bolt to lock

Could be a few things,lock not lining up with the keep,lockcase could be broken and need replaced,difficult to diagnose with out pics,or knowing the type of door.

You haven't specified wether this is a upvc or not. Either way the only thing stopping the bolt fully engaging will be that the bolt is not lined up correctly with the keep. A common problem on upvc doors where the doors have dropped. If it's a wooden door the easiest way to confirm the diagnosis is to try locking the door with the strike plate removed.

Could be out of alignment.. if it’s a pvc door it could have dropped. If it’s composite or wooden it probably is the strike plate not being aligned with bolt .. basically it’s a door realignment problem..

Contact and use a proper locksmith

As others have said, not enough specific information provided. However, assuming this is either a upvc or composite door it's usually 1 of 3 things. 1) A new lock cylinder/barrel is needed - (this has already been tried, so rules this out) 2) The door needs realigning/adjusting 3) The locking mechanism has failed/is faulty & needs replacing A good test to check to see if it's an alignment issue or a faulty lock mechanism is to go through the locking operation when the door is open. So firstly open the door, then lift the handle to see if the locking points on the mechanism engage (move up/down or hooks/bolts come out, or a combination of both). If they do, then turn the key in the barrel. If everything is working correctly the mechanism should now be deadlocked & you won't be able to pull the handle back down, until you turn the key the opposite way to unlock it. If you can lock it with the door open, as described above, this usually means the door needs realigning, but if it still doesn't lock when the door is open it's usually a case that the mechanism is faulty & needs replacing. Whichever the case, I recommend you employ the services of a Locksmith to rectify the issue.

Could be that gearbox has failed and then broken euro.