I certainly wouldn't propose to think I know more than your gunsmith, but having had similar problems over the years I can suggest that there are numerous things that can/will cause this.
I would think that your gunsmith would have thoroughly cleaned the action innards of any stiff gunk that may have accumulated over time. If not, a hot bath in an ultrasonic cleaner with a bit of Dawn dish soap, then a thorough drying and proper lubrication can work wonders. Then, check for free, unhindered movement of the hammers.
Another consideration is that of the chamber rim cuts. I have some vintage guns that have deeper rim cuts than others and it allows certain brands of shells to sit a bit too deeply in the chambers and the strikers just can't get to them as well, resulting in light strikes.
In that vein, have you tried various shell brands to see if your problem is limited to one or two brands, or if it occurs with all brands? I have found that B & P Comp One loads have a different shell head rim thickness, or shape, that allows them to sit deeper in the chambers. I only have a couple guns that will fire them 100% of the time.
One other possible culprit is that the gun may not have sufficient firing pin protrusion through the standing breech. If you have the means to do so, hold the hammers at maximum contact with the back of the standing breech, where they fall, and measure the amount of protrusion through the breech.
Again, all these things should have been checked by a competent break-action 'smith, but it's where I'd start anyway. Good luck.
