Let's say you're in charge of two projects: moving a large boulder that got stuck and traveling 1,000 miles on foot. I know, I know, these are strange projects. But let's run with it to illustrate one of the reasons why projects fail.
Both projects require a lot of effort. But how to execute them varies greatly.
If you walk by the boulder every day and give it a light push, it will move nowhere. Even if you do this every day for a year and try really hard, the boulder will not budge. Incremental, small steps will lead to failure. If instead you gather the troops and meet at the same time for one monstrous push, the boulder will move.
Now imagine gathering the same troops and meeting up to take a single step together for the 1,000 mile project. Where will that get you? Exactly one step from where you started. You will have missed the goal by 1000 miles minus one step. But here incremental, small steps are perfect. You can take a series of small steps each day, chart your progress regularly, and estimate how much time remains at your current rate.
Traveling a great distance on foot can be done in a series of small steps, but unsticking a boulder requires a single motion that is coordinated and focused. Figure out which type of project you're facing.