Is this a single or double set trigger? Without more info and/or photographs it is hard to tell, but a first guess would be that your trigger is in contact with the lock’s sear arm before the lock is at half cock. Take the trigger out of the gun, then see if the lock goes into full cock. If it does, try shimming the trigger. If that works, you could make the shim permanent, or take some metal off the top of the trigger. Just a guess based on limited information you provided.
I wouldn't mess with the trigger mechanism until other things have been checked out. Sometimes just a cardboard shim to decrease the depth of the trigger inlet will do the job. Also check carefully that the sear arm and all of the lock's moving parts are not contacting wood. I have found that most of the time such problems are a result of faulty inletting. Fortunately, it's the easiest thing to fix.
A piece of popsicle stick or similar works great. Usually fills the slot, and once you figure out the thickness (number of pieces and/or thinned down) pieces you can glue them in place. If you don’t glue them in place, they can fall out when you or someone else disassembles the gun, and exactly where they go is forgotten. I have seen loose shims forgotten about more than once.
That's what I've done with several. It's easy to see the "cavity/inlet" for the trigger mechanism, where as the inlet for the trigger plate is a separate shelf.
How does the lock work when it's out of the gun?
Look at the lock mortise. If you see Shiney spots the lock components are rubbing on the wood. Take a sharp chisel and remove the shine and you should be okay.
