Lack of visible results
Even if there is an end goal towards which you are working, it is necessary to get regular feedback for your work. A lack of visible feedback/results of your efforts combined with Lack of challenges can result in a massive productivity decrease.
The factory line effect
This happens when you have an end goal defined, and a somewhat clear process and steady inflow of tasks to do. You keep on doing tasks as they come in. Once a task is done, you move on to the next task in the production line. Now if you don't have a system to get some feedback or to see how your work is contributing towards the end-goal, you might end up feeling like a factory worker who is just mindlessly doing something repeatedly. Even though your tasks are not repetitive and even if they involve some level of creativity, your process might make you feel exhausted and unmotivated.
A task is not done once you finish doing it
A task is done when you actually see the result of the task. It is important to make sure you have a system set up for seeing the results of your work. Set up some analytics, set up a system to collect and process user feedback and make sure that observing the results and consuming the feedback is a part of your workflow.