A small-time contractor in a nearby city uses another contractor's federally registered trademark to advertise and provide services. Customers confuse the small-time contractor with the trademark owner. They complain to the state contractor board. They call the trademark owner and complain about service. The trademark owner is livid. It hires an attorney to send a cease and desist letter. The small-time contractor ignores it.
A consultant provides services to a client. There are a few minor issues during the project, which the consultant promptly fixes, even though they weren't her fault. The consultant discounts her fees as a courtesy for the inconvenience. The client doesn't pay the consultant's fees and stops responding to the her emails and phone calls. The consultant hires and attorney to send a demand letter for the unpaid fees. The client ignores it.
In both instances, the lawyer's client thought a firm letter from a lawyer (on the lawyer's letterhead) would change behavior. In both instances, they were wrong.
Why didn't the lawyer's letter work?
Most people these days don't seem to be afraid of legal action. Perhaps legal action is so prevalent, people are desensitized to the possibility. In other instances, people are being pursued by so many creditors, they give up on responding and trying to work things out. In still other instances, people don't believe you will do anything to enforce the threat - they dare you, they call your bluff.
A lawyer's letter is a valuable tool, but, in my experience, rarely changes behavior if it's the only tool used. Instead, the lawyer's letter is just a step in the process. Before the letter is even sent, you need to be ready and willing to file a lawsuit. Then, you need to file that lawsuit when no response comes.
In the first instance above, the trademark owner did not want to file a lawsuit. The letter didn't change the small-time contractor's behavior, so the alleged trademark infringement is ongoing.
In the second instance, the consultant had her lawyer file a lawsuit shortly after it was clear the client wasn't going to respond to the demand letter. The client didn't respond until the day before trial, when it paid the past due fees in full. The lawsuit changed the client's behavior, but the demand letter notified the client of its possibility, which I believe gave the client enough time before the trial to evaluate its position and conclude it should settle.
Have your lawyer send the letter on her letterhead, but realize that probably won't be enough to change behavior. Be resolved to file a lawsuit if there is no prompt response. Then don't wait long to do so when no response comes.