Four thoughts. First, define what maximum and minimum effort estimate is acceptable - for example, tasks should not take less than 2 hours or more than 4. Second, communicate that contingency shouldn't be included in the estimates so they aren't padded. Instead, have some room in the overall plan for tasks to slip. Third, measure actual vs. estimates to learn how the team estimates - you'll find that some people consistently over estimate, and some consistently under estimate. Show them and have them adjust course. Finally, make sure that the people estimating understand that no answer is acceptable - it is better to redirect a task than have wide guesses.