What should a tax advisor do about clients who did not return?

Contact them in the quiet months rather than the following January, and treat it as a scheduled task rather than a reaction.

Most practices notice the absence in February, when there is no time to do anything about it, and by then the client has already filed elsewhere.

A better approach:

• Run the list in June. Compare last season's clients against the prior season and identify who did not come back.
• Send a short, non-defensive message. Something that asks whether anything changed and makes it easy to say they went elsewhere, or that their situation changed, or that they simply forgot. A meaningful share fall into the last category.
• Offer a specific reason to reconnect, such as a mid-year planning conversation, rather than a general invitation.
• Learn from the replies. If several cite price, or slow communication during the season, that is useful information you would not otherwise get.

The people who left for a reason will usually say so, and that costs you nothing. The ones who drifted are recoverable, and they are cheaper to win back than a stranger is to acquire.

Doing this in June also means you have the time to have the conversation properly.