As
@redndkhaki said, make sure your TMs know how to perform the task the correct way. Identify your underperforming TMs and identify any potential gaps in training. Explain it, do it, have them do it. After you do this, reflect with them on how it went. This is another opportunity to correct behaviors on the spot.
Once they have been shown how to zone correctly, hold them accountable for zoning correctly 100% of the time. My TLs (well, most of them) all recognize that some days, you just won't finish. Perhaps you were swarmed by guests. Perhaps your zone had a visit from the Tasmanian devil. But where "follow up" often is neglected is it's not a "one and done" approach. In order to be an effective leader and drive your team for results, you need to follow up continuously on your team's performance. Recognize your team for improvements in performance, seek to understand no improvements. At first, you may have some unfinished zones, but continuous follow-up will allow you to identify consistent opportunities and give you a chance to ask "I've shown you how to do this and you've been given sufficient time to improve. Why isn't it being done? What's preventing you from doing (thing)?" If there is a recurring reason of "too much workload" try switching zones and put a different TM on that zone for a bit, one who has also been shown proper zoning. My TLs often put team trainers in this spot as part of their role in training. You can then use this as insight to the zoning process. If the new TM can finish the zone consistently, it's time to hold the other TM who is having problems finishing accountable for this. "If other TMs can finish your zone, there is no reason you shouldn't be able to." However, if other TMs also struggle with this zone consistently, perhaps that means the leadership team should evaluate its zone breakdown and see if there is a way to more effectively leverage your team. (NOTE: It can also mean your team has significant issues with performance.) My store has done this a few times, and it may change 3 or 4 times every year depending on seasonality.
TL;DR: Ask questions. Identify and correct wrong behaviors. Continuously follow up on your team's performance. Recognize improvement, coach non-improvement or deterioration. Use TM performance on zones to evaluate effective use of resources and identify zone assignment breakdown opportunities.
