make sure you cleaned off any packing grease from the manufacturer, and lube the frizzen screw with a good quality machine oil. As Jason said the frizzen arm could be rough where it contacts the frizzen spring, and it may need just some light polishing.
However, if it works ok from clean and then gets sticky, it sounds like it is dry and needs lube when you starts out and fouling is causing it to stick more. Try a slight dab of grease where the frizzen arm contacts the frizzen spring. See if keeping it lubed while shooting, at both the arm and the pin, does any better.
A drop of oil on the pin every five or six shots. Wipe off any excess from the frizzen face immediately, (with alcohol) it will interfere with sparking.
When the frizzen is flipped back and forth, there should be no gritty or grinding. No marks should scratched onto the frizzen spring. If marks appear in the metal or there is a grinding/gritty sensation, it needs a light polishing. Usually the springs are already smooth and there is most likely a burr from wax casting on the frizzen arms that needs to be polished. That can be often handled with as little as a single pass with an very fine sharpening stone. There is a fine toggle/cam action to the way a frizzen arm works and you don't want to change any lengths or angles.
If you are not experienced with dissassembly of the lock, find a seasoned person with the spring vise to help you. It is easy to destroy a lock with improper tools.