Bob,
There are a number of possibilities that might be tried. First, look to see that the flint is installed correctly. At half cock the flint edge should be just short of the frizzen. If the flint is too short it may not give the frizzen enough push. You may want to try the flint both bevel up and down. Some locks show a preference. Also sometimes poor lock geometry can be minumized by reversing the flint.
Check to see if the frizzen is bound up in some way. With the frizzen spring removed the frizzen should flop back and forth freely. Make sure it doesn't leave a wear mark on the barrel. Make sure the frizzen screw isn't too tight.
The foot of the frizzen may be rough where it contacts the frizzen spring. If it is, one can use a stone to smooth it. Don't change the shape of the foot just yet; just take the rough spot away. BTW with the frizzen and spring off the gun you can look for wear marks where either may be rubbing on something.
The frizzen spring may be too strong. Springs are generally weakened by taking off material from the back edge, the side that goes against the the plate.
The lock may be poorly timed. If the flint is allowed to slide slowly down the frizzen face the frizzen should kick open when the flint is at the bottom of the frizzen face. If the flint gets to the bottom of the frizzen and it doesn't open, the frizzen foot may need to be reshaped. You want to rule out all the other things mentioned above before resorting to this. Watching the frizzen contact the spring as it opens may give you the idea. However . . .
Reshaping a frizzen foot is an acquired skill - I don't do this. When I rule out all the other things, I take the lock to a friend with better lock-tuning skills that I have.
Good luck with your lock. You may have found out why the former owner decided to sell it. By trying a little harder that he did, you may find the solution which he did not. It could be quite simple.
Regards,
Pletch