Perhaps it's because you're not putting things in sequence. Just having a list of things to do is not enough. You must also decide in what order you will get everything done. Something must be done first, and then something else after that, and so on. Certainly you want to make sure the most important tasks are near the top of the list.
Many people also find it helpful to do the least desirable things at the beginning of the day.
And of course your appointments and commitments to others will also determine your schedule. Beyond that, even an arbitrary sequence is much more effective than no sequence at all. Sequencing your tasks strengthens your commitment to getting them done. Success comes from taking one step at a time. The more clearly and specifically you can lay those steps out in front of you, the more you'll get accomplished.