If you've managed people you know that one of the most common traits most employees share, is that they'll almost always do the easier tasks first.
That's just human nature. Now, it's not true for everyone, but most people prefer easier work because a sense of accomplishing something tough isn't worth as much as a sense of well being and lack of stress. You can eliminate this problem by carefully structuring your team's work day using scheduling software.
One great trick is to assign the most difficult tasks to your team first, and follow with the simpler tasks afterwards. Why is this an awesome strategy? One, it will get employees to do the hardest work when they have more energy. In the beginning of the day, they are fresh and tough projects will not seem as hard. Further, they will put more effort to do the work and the quality will be higher which increases your chances of overall success.
Second, having to do easier work at the end of the day helps de-stress them and creates feelings of greater job satisfaction. Think about it, would you rather be worn out and tired at the end of the day because you've just finished an incredibly difficult project, or would you rather do simpler, easier work for the last two hours of your work day? Now, the amount of work doesn't change, but by changing the progression of the tasks, you're changing your team's perception of the difficulty. And ultimately, a happy team is one that will go the extra mile for their leader and your team members will work well not because they're forced to, but because they feel a sense of accomplishment.