There are times when I have to really slide my one living room window open and slam it a bit to make sure it is fully seated so it will lock. Nothing in the track and I've tried to put a little lube on it, but it still is tight in the tracks and takes a solid push to move it to open or close it.
I guess what I'm trying to say is, are you sure you have it fully seated so the lock is actually locking? If there's nothing in the track to stop it, it might be like mine and is just a bit too tight in the frame.