Misfire only on double action? You could try using Federal primers, they take the least amount of energy to ignite them. Otherwise, I expect someone did a trigger job and decreased the mainspring pressure. They may have changed the bend in the stock spring, perhaps ground it down. If it is the stock spring you might be able to bend the upper arm of the spring just a little and regain the factory tension. Bend it at a point approximately 7/8" to 1" from the middle of the stirrup seat.
If it is an aftermarket spring, you may have to buy a new spring. If you are uncertain of how to proceed, you can take it to a gunsmith who is thoroughly familiar with older Colt revolvers or send it to Colt for service.